As a Mandarin/Cantonese bilingual speaker, I would say don't worry about this line in the video, because I've never seen a sentence goes:
Noun1 莫过于 Noun2
instead, there is usually a 'most' ‘最’：
Most adj. something 莫过于 Noun2
Literally translated (to maintain the word order) as,
Greatest pain, (can) not surpass, (the situation) when learning Chinese, the dictionary and daily life expression have discrepancy.
where English words in the brackets are those not appearing directly in the Chinese sentence, but added to make the translation grammatical.
And not == 莫； surpass == 过于.
For these two examples, I personally think the first one sounds very unfamiliar to me, while the second sounds more natural (probably because it is more of the 文言文 style).
If they are expanded according to the rules I summarised above:
快樂莫過於知足 --> （最大的 or 最简单的）快乐莫过于知足
不知羞恥，莫過於此 --> （最过分的 or 最令人不齿的）不知羞耻 （的行为），莫过于此
The problem arises from the discrepancy between word-by-word translation vs. sense-by-sense translation.
The direct translation should be:
莫 == not
过于 == surpass; where 于 is a preposition for 过, combined meaning beyond/over/surpass etc.