score:-2
Eastern Orthodox Christians do not necessarily fast on Mondays. It is up to a believer to decide whether to fast on Mondays and his motivation to do that is also his own.
And Christ surely didn't meant fasting when He was speaking that verse.
Upvote:1
Monks fast on Mondays because Monday in the daily liturgical rite is dedicated to Angels and monks conduct "angelic life". Current Orthodox liturgical rite originates from VI century.
The link that you provided is definitely wrong with the aim of fasting, it is not to overcome someone in merits (as Pharisee thought in the parable about Pharisee and Publican) which is obvious pride but to repent of sins.
This site (Russian only) has a collection of interpretations of Matthew 5:20 from 14 Orthodox Holy Fathers and modern theologians, none of them mention fasting.
If some lay person with the blessing of his spiritual father takes the feat of fasting on Mondays - that is not delusion if he keeps the mentioned repentant disposition of the soul and does not think that he does something special. St. Nikodemos writes about fasting on Mondays that "those among the lay people who want to fast this day are praised before the Lord and will get appropriate retribution" (link).
In the Patericon of Volokolamsk (Russian), there is a story when two brothers, noble men, were captured by Tatars and one of them was fasting on Mondays even in captivity. Once an Angel appeared in front of him in the form of a man. The Angel took him and walked with him through the Tatars without them noticing anything. When Tatars found out that he disappeared, they killed another brother. When the man was saved, he asked Angel: "Who are you?" - "I am the Angel of Monday". The man later became a monk.