Why not?
Emma Frost is Emma Silverfox, now going by her original template's surname. She has been created in 1965, so the time travel in DOFP did not alter her birth.
Sean Cassidy is the clone of the original Banshee, who already appeared on ORIGINS (set in 1983). He has been created before 1973 as well.
The entire movie could be set in 2002 or so, some time before "The New Mutants" (set in 2008), in an time where Xavier and the X-Men were far from the school for some reason.
Jubilee could be the very same Jubilee from X1/X2/X3, related to the original Jubilee seen in "Apocalypse" (set in 1983).
I never watched the TV movie, so I don't know if any detail or event or character on it contradicts the mainstream X-Men timeline. Hopefully not. Tell me if that is the case.
GENERATION X:
Jubilee
M
Mondo
Skin
Buff
Refrax