I read some theory not too long ago that X-Men 97 could tell the story of how Earth-199999 "split off" from a branch somewhere adjacent to (comic) Earth-616 to it's own multiverse towards the beginning of the time onwards - involving Immortus' machinations from the original show
https://marvel.fandom.com/wiki/Nathaniel_Richards_(Immortus)_(Earth-6311)#cite_ref-73
Maybe that's what Kevin Feige meant by Earth-92131 being "MCU" even though it's clearly not? Telling the backstory of how this "He Who Remains" came about?
(All of this is pre-birth of the MCU, in totality, as in pre-Iron Man 1.)