This isn’t true, my friend! You can temporarily hide a unit from FE7’s prep screen by giving or removing the Escaped flag before the chapter ends. Doing it in the middle of the beginning scene might not address a secondary flag that comes into play, but there’s a way around that. l go a bit more in detail about it here: How to bar a unit from a chapter? - #14 by Kemui52
I’ve also been told REMU and REVEAL function the same way for FE8, though l’ve yet to look into that personally.