That rumor sounds like what could be a deadpool style two second joke.
F4 movie is coming out in like 2023 with Jon Watts (Spider-Man Holland Trilogy director) directing. They're definitely doing a full recast. I think the best part of getting F4 into MCU pretty quick is getting a hopefully legit good Dr. Doom in MCU. He can definitely pull some movies as a Thanos like character.
I think I saw some rumor that Chris Evans might come back at some point as Captain America in some side thing like a TV show that takes place in the past.
And yeah, the multiverse
has to be how they bring Mutants to the MCU. Mutants in MCU is happening down the line and Mutants/X-men have too much backstory to just start fresh as oh no Mutants are first appearing in 2023 or something. Multiverse lets them start way down the line and just explain what a mutant is and what happened in their world real quick. The end.
I think Hugh Jackman is the only character
spoiler (click to show/hide)
outside of Quicksilver
That they may not recast when Mutants hit MCU. Despite what Jackman said in the past and that he's getting older, Marvel has $$$$$$$$$ money and Jackman likes playing the role. I can see Feige making it happen though I bet if Jackman is Wolverine in MCU it'll just be a small role where he passes the torch to X-23 as his successor and is more a mentor character. That way he doesn't have to spend much time in the role or do heavy action stuff as he gets older but can still appear occasionally for small bits.