Lol, why not? Glimpses of their potential have been shown numerous times over the years. Xavier determined they were Omega when they were still early teens...and even of you thumb your nose up at that as a retcon, here is an original panel from the original 60s run of the X-Men that has a villain plainly state Jean's potential
Screenshot_20210617-200057_Marvel Unlimited.jpg
Before Phoenix, before X-Men Forever