I think "all the things I mentioned before" sounds perfectly fine and the sense is essentially the same. The difference between past simple and present perfect is often quite distinct, and usage of one or the other can completely change a sentence. I keep repeating this sentence in my head though, and the only difference I can think of is tiny.
"All the things I mentioned before" sounds like the speaker stopped mentioning things you need to consider at some point in the past, even if it was a short time ago or a couple pages prior.
The present perfect always ties things in to the present, so here "All the things I've mentioned before" makes it sound more recent, even if only by a small amount of time, like they could have mentioned something you need to consider right before saying that sentence (which it looks like the writer actually did in the paragraph you posted.)
(not a teacher, just a language lover)