I don't get it. Just because the X-Men are being re-introduced in the MCU doesn't mean they can boot them into the main MCU world via multiverse. I think cos Feige wants to use the multiverse to bring a version of the 2000s X-Men with the MCU characters for SW so at least he could finally accomplish a Bryan Singer X-Men/MCU Crossover for fan service before they leave and then afterwards, the heroes go back to the main MCU timeline and they meet their X-Men from somewhere idk maybe Xavier reveals their existences later, afterall there was the She-Hulk reference of Wolverine. But hopefully they don't actually place a variant X-Men world into the MCU because it wouldn't feel exact and perfect to be a true MCU X-Men.

My explanation and it makes the most sense

I think they're using the multiverse since X-Men actors cannot be recasted until 2027. So they'll have to wait until after Secret Wars, that's why people speculate the multiverse will boot the X-Men into the new universe post Secret Wars.