I think they'll remember a few more things, like:
That Three Houses puts a much greater emphasis on things that happen out of combat.
That Three Houses makes a big deal out of building your characters up, vs Engage encouraging you to swap out old units for newer ones.
That anyone can use virtually any weapon in Three Houses, vs units being stuck to specific weapons in Engage.
That Break pushed them to think about what weapon they had equipped, even if they didn't take much advantage of it on their own turn.
They'll definitely remember Engaging with Emblems and getting super attacks in Engage, combat arts being the closest equivalent in TH but noticeably different.
If they used said combat arts, they'll likely remember how durability impacted things in TH, vs not existing in Engage.
If they played the same difficulty on both games, they likely found Engage harder. If they jumped into Engage hard mode because they played TH on normal and thought that meant they were prepared, they likely found Engage a lot harder.
You don't have to go into the nitty-gritty of what stat boosts each Emblem gives, or the best skill builds, or how monster mechanics work to notice obvious differences in the games and how they play. Maybe if you're talking about 12-year-olds they'll have more difficulty remembering differences, but this isn't Pokemon.
I definitely noticed stark gameplay differences the first time I played Engage, on hard mode.