It's come up a few times since Friday. I wrote and deleted a thingie but here's my take on it again, for interest.
Short version: no official word, but it's an uncanny (hah) coincidence, and the happy sort that's wide open to reframing as deliberate the road so long as nobody comes out and says it wasn't deliberate too soon in the long process of figuring out where any number of franchises might converge in the anything's-possible DisneyMarvelFoxverse. :)
What we know about Luke Roessler:
He's vetted, i.e. an active and reliable talent that's made it through the child casting gauntlet and, more importantly, has proven a reliable an agreeable cohort (in terms of you hire a child, you gotta work with their parents, handlers, that whole complicated relationship). I'm the sibling of a now-grown child performer and, while I'm no authority, can bear witness to the importance of being an agreeable cohort. The kid can't just be on time and talented, in fact that's pretty far down the list. Relationships between everyone must facilitate the smooth operation of the entertainment machine and if you gum it up, like being a troublesome momager, the gigs go away very quickly.
Roessler is a Vancouver native and Legion S1 was of course filmed there. Deadpool 2 was also filmed there. Ryan Reynolds is a fellow Canuck and known to be very hands-on with the whole DP franchise. Whether he or anyone from casting or production on DP was involved in Legion S1 is unknown but on top of your DisneyMarvelFox observation, it seems reasonable that someone somewhere would have seen Roessler and shortlisted him.
Interestingly, Roessler's latest Insta (highly regulated and for publicity only, it seems) announces both he and the kid who played even-younger David (the one blowing bubbles) and are now part of the Disney stable, so you can expect to see them pop up here and there for years to come.
Roessler's also been on episodes of "Arrow" and "Supergirl", admittedly in one-off scenes, but that supports the argument there's a smaller selection pool for kid actors than we tend to think and that his casting in anything is coincidence. Though I'm happy to buy an extra roll of tin foil for the hat that picks up evidence of a possible Marvel vs. DC (TV) brawl brewing. Heh heh.