It’s not official but I’m 99% sure that’s the case. There has to be some major reveal with each big Marvel film and FF sounds perfect for Infinity War. Besides, trading all X-Men television rights for just the Skrulls? That wouldn’t exactly be equitable and Marvel doesn’t exactly have a weak bargaining hand. X-Men television rights is essentially useless to Marvel at this point without movie rights and, especially since the success of Deadpool and Logan, Fox will never give up the property. And the multiple failures of Fox to launch FF make that property worthless to Fox except as a bargaining chip. I’m 99% sure FF got traded to Marvel - everyone wins in that exchange.
Well, you said it yourself: X-Men television rights are essentially useless to Marvel at this point, so trading something useless for the rights to use certain villains/characters in future movies? Sure, seems like it could be fair trade. Would Fox trade the rights to an entire movie franchise for television rights? I don't know... if you're a Monopoly kind of guy, that's like trading Connecticut Avenue for Park Place.
I'd love to see the Fantastic Four in the MCU, and I'd like to emphasize that I REALLY hope you're right here, but I'm not sure how realistic it is. Plus, I feel like we would have heard something about it by now if it did happen. The Sony Spider-Man deal was announced pretty much right after it was made.
But still, fingers crossed.