The thing is, the “phoneme” [sic] za has an array of meanings, and this was true already by the Proto-Slavic stage. Sure, one could recommend as a mnemonic device that a learner see Russian zapomnit’ ‘commit to memory’ as a compound of ‘behind’ + ‘remember’, but that isn’t the actual etymology of the word. It is a folk etymology.
It is also possible to demoralize and silence newcomers by taking every opportunity to show how far you are above them, and how unworthy they are of your superior knowledge base.
One of those approaches leads to more productive thought and breakthroughs than the other.
You’ll find that HN tends to react badly to inaccurate posting on any scientific field, and one of the great things about this venue is that there are many trained people in various sciences who can call out wrong as wrong.
In the case of Russian, the za here is not a preposition literally meaning ‘behind’ but rather an already fully grammaticalized marker of inchoative aspect that one encounters in a number of other verbs like засыпать ‘to fall asleep’. Russian запомнить is analyzable as ‘start’ + ‘remembering’.