What is the meaning of this short passage in Thomas Carlyle's 'French Revolution'?

Taking a single statement, completely out of context, will rarely helpful in understanding the meaning of the statement.

So the first step is to collect the surrounding text, in the hope that references exist that will point to sources informing us of who, when and where the statement is referring to. This especially true when persons and historical events are referred to, that at the time were common knowledge, but today less so.

In this case we have

• Duke d’Aiguillon 1720 - 1788
• Battle of Saint Cast 1758
  • French victory against the British

As the French commander, Duke d’Aiguillon was therefore the victor of the battle.
French sources claimed that during the battle Duke d’Aiguillon was being entertained by the miller at his headquarters.

sharply as thou didst, from the Mill of St. Cast, on Quiberon and the invading English

• while observing the battle from inside the mill, where he was being entertained

thou “covered if not with glory yet with meal!” Fortune was ever accounted inconstant: and each dog has but his day.

• he got lucky to get his day of fame, despite not having done anything to earn it (but at least got a meal)

This is, at least, how I would understand it based on the found context below.

The French Revolution a History Volumn 1, Chapter 1, Page 4 and 5

DEATH OF LOUIS XV.

So stands it written; in lasting memorial of that year 1744. Thirty other years have come and gone; and “this great Prince” again lies sick; but in how altered circumstances now! Churches resound not with excessive groanings; Paris is stoically calm: sobs interrupt no prayers, for indeed none are offered; ex- cept Priests’ Litanies, read or chanted at fixed money- rate per hour, which are not liable to interruption. The shepherd of the people has been carried home from Little Trianon, heavy of heart, and been put to bed in his own Château of Versailles: the flock knows it, and heeds it not. At most, in the immeasurable tide of French Speech (which ceases not day after day, and only ebbs towards the short hours of night), may this of the royal sickness emerge from time to time as an article of news. Bets are doubtless de- pending; nay, some people “express themselves loudly in the streets.” But for the rest, on green field and steepled city, the May sun shines out, the May evening fades; and men ply their useful or useless business as if no Louis lay in danger.

Dame Dubarry, indeed, might pray, if she had a talent for it; Duke d’Aiguillon too, Maupeou and the Parlement Maupeou: these, as they sit in their high places, with France harnessed under their feet, know well on what basis they continue there. Look to it, D’Aiguillon; sharply as thou didst, from the Mill of St. Cast, on Quiberon and the invading English; thou “covered if not with glory yet with meal!” Fortune was ever accounted inconstant: and each dog has but his day.

Emmanuel-Armand de Richelieu, duc d'Aiguillon in the context of the

Battle of Saint Cast

During this time Richelieu, duc d'Aiguillon, military commander of Brittany, had gathered some 12 regular line infantry battalions, including the Regiments of Royal Vaisseaux, Volontaire Étranger, Bourbon, Bresse, Quercy, Penthièvre and Marmande, from the garrison of Saint Malo a brigade of the Regiments of Fontenay-le-Comte, Brie and Boulonnais; six squadrons of cavalry, some companies of coastal militia, and several artillery batteries. The French army amounting to 8,000 or 9,000 men, under the field command of Marquis d'Aubigné, was fast marching on Saint Cast from Brest by way of the town of Lamballe and from the town of Dinan.
...
While the British continued such expeditions against French colonies and islands beyond the reach of the French land forces, this was the last attempt by an amphibious expedition in force against the coast of France during the Seven Years' War. The fiasco of the embarcation from Saint Cast helped convince British Prime Minister Pitt to send instead military aid and troops to fight alongside Ferdinand and Frederick the Great on the continent of Europe. The negative potential for another disaster and expense of expeditions this size was considered to outweigh the temporary gain of the raids.

The French had this to say about their own performance:

"si les Bretons s'étaient couverts de gloire, le petit Duc (d'Aiguillon) s'était couvert de farine." (though the Bretons were covered with glory, the little duke was covered with flour.) This refers to the location of the headquarters at the mill of Moulin d'Anne, where it is rumoured that the duke was entertained by the miller.

The Wikipedia article seems to confirm that during the battle, for which Duke d’Aiguillon is famous for

• he was actually busy else where, being entertained by the miller

He [Duke d’Aiguillon] died, forgotten, in 1788

After this major 'achievement', no further claims to glory were remembered.

Forlorn enough languished Duke d’Aiguillon, some years ago; covered, as we said, with meal; nay with worse. For La Chalotais, the Breton Parlementeer, accused him not only of poltronery and tyranny, but even of concussion (official plunder of money); which accusations it was easier to get “quashed” by backstairs Influences than to get answered: neither could the thoughts, or even the tongues, of men be tied. Thus, under disastrous eclipse, had this grand-nephew of the great Richelieu to glide about; unworshipped by the world; resolute Choiseul, the abrupt proud man, dis- daining him, or even forgetting him. Little prospect but to glide into Gascony, to rebuild Château there,* and die inglorious killing game! However, in the year 1770, a certain young soldier, Dumouriez by name, returning from Corsica, could see “with sorrow, at “Compiégne, the old. King of France, on foot, with “doffed hat, in sight of his army, at the side of a “magnificent phaeton, doing homage to the — Dubarry.”