The idea is that you were "given" that money when you got the loan and you weren't taxed on the value of the loan when it was provided to yoy, so you can't deduct it when you pay it back.
Maybe it's a bit like when people talk about the "good old days" of xyz music genre, and how music was better back in (insert year) compared to (current year).
The only songs from the 70s/80s/90s that still get play time are the top 10% songs from those eras and everyone forgets about the other 90% of crappy music that played on the radio.
The only thing I really miss from Reddit is a few of the smaller, niche subreddits that had small but active userbases. But that will come with time as the Lemmy userbase grows.
The idea is that you were "given" that money when you got the loan and you weren't taxed on the value of the loan when it was provided to yoy, so you can't deduct it when you pay it back.