Was I incorrectly denied boarding due to passport extensions?

No, you were correcly denied boarding at the gate.

Until 2018, the UK issued passports that were valid longer than 10 years (the remaining time of the previous passport was added). For new passports this should no longer happen.

Before 2021, for UK citizens, only proof of EU citizenship was required, so it was not important that the validity of the passport was older than 10 years.

Since 2021-01-01, UK citizens must fulfill the conditions set out in Article 6 of the Schengen Border Code, since they are now considered third-country nationals.

Article 6 Schengen Border Code
Entry conditions for third-country nationals

1. For intended stays on the territory of the Member States of a duration of no more than 90 days in any 180-day period, which entails considering the 180-day period preceding each day of stay, the entry conditions for third-country nationals shall be the following:
   (a) they are in possession of a valid travel document entitling the holder to cross the border satisfying the following criteria:

• (i) its validity shall extend at least three months after the intended date of departure from the territory of the Member States. In a justified case of emergency, this obligation may be waived;
• (ii) it shall have been issued within the previous 10 years;

This is based in the ICAO Doc 9303 (PDF) recommendation for Machine Readable Travel Documents that they have the :

• capability of data retrieval for 10 years, the maximum recommended validity for a travel document

Until the European Commission has clarified the 10-year rule, one should assume that everything within Article 6(1)(a)(i,ii) defines what a valid travel document is while within the Schengen Area.

For the Schengen Area in general: Entry requirements - Germany travel advice - GOV.UK: valid for at least 3 months after the day you plan to leave Germany, or any other Schengen country ; The 3 months you need when leaving a country must be within 10 years of the passport issue date.

Update:

2022-04-22: Hampshire family fall foul of post-Brexit passport rule - BBC News

...
Nina Gurd was told her passport, which expires next February, was not valid for travel to Portugal.

She went to the airport, confident she had more than the required three months before the expiry date which had been extended when she renewed her passport.

But she was told the expiry date was irrelevant, she told the BBC.

"The lady at Bournemouth Airport said it needs to be within 10 years of the issue date," she explained.

Mrs Gurd's passport was originally issued on 29 May 2012, meaning it would have been due to expire next month.

But when she renewed it early, another nine months were added, giving it a new expiry date of 28 February 2023.

Mrs Gurd, her husband John and their three sons Jack, Harry and Charlie, were forced to return to their home in Botley, Hampshire, missing out on their holiday in the Algarve.

Mr Gurd said: "When we were booking our holiday, we were only ever asked for our passport numbers and the expiry dates, nothing else.

"But the expiry date is apparently meaningless."

...
But Mr Gurd said the advice was "all incredibly vague".
...

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2022-04-25: Airports wrong to turn away holidaymakers, EU says - BBC News
Once the three-month expiry buffer is taken into account, a passport needs to have been issued no more than nine years and nine months ago.

However, the EU's executive arm has now said a "more generous interpretation of the rules is possible".

It said in a statement: "Our previous advice was intended to make sure travellers are prepared to comply with the most stringent possible interpretation of the Schengen rules."

Requiring travellers to have passports less than 10 years old throughout their stay provided a guarantee the documents would not be refused at entry, it said.

But "based on the experience gathered in implementing these rules" and in discussion with member states, it was "now clear that a more generous interpretation of the rules is possible and we have therefore updated our advice".

Entry should be allowed to those travelling with passports issued within the previous 10 years at the moment of entry into the Schengen Area, it added.

Note: the article did not supply a link to this statement or updated advice.