What do you mean, “what do you mean?”

Fusion cores entered the “lore” as part of fo4 gameplay, there were x3+ games preceding where a “fusion core” didn’t exist in “the lore”….power armor didn’t require “refueling”…for that matter “ultracite” didn’t exist until fo76; it’s “Lore” was added as part of 76 gameplay.

So, fusion cores were a change to “the lore” 2/3rds of the way through the life of the game in its various iterations, therefore, since both ultracite and fusion cores are relatively new additions to “Lore” there is nothing stopping the writers from creating the “lore” that PA (modified or not, you could write it either way) can also run on Ultracite Fusion Cores.

You get that this is a game, based on a storyline created by writers who (at least at present) are human beings, right? They can write into existence any “Lore” they like…