What you're saying here is nonsensical. Bendis could easily have them say something to the effective of being inspired by Jon's father, but wanting to take Jon because of the monumental moment of him creating the UP, but he doesn't. He has them say
"we dedicated our lives to the ideals of superheroes in direct honor of this day...this--the greatest day in the age of heroes". And then to top things off in issue 16 of Superman Clark literally says that he was never given the same opportunities as a kid that Jon is being given with the invitation. That's Bendis doubling down on the idea that it's Jon's Legion.
Then to add on to all of that, Bendis flat out said that this is the first time anyone is meeting the Legion in this continuity, and that this is an outright reboot so he's doing an Ultimate version of the Legion.
But funny enough, when someone asked Bendis if the Kents were alive he THEN decided to play coy and say that he'd have to wait for Doomsday Clock.
The intent on Bendis' part is clear.
Well, we'll see what issue 4 of Legion of Super-Heroes has to say about it. That's gonna be the origin story of the Legion. We'll see how old they were and what they looked like before coming to get Jon.
But I'll just say that from the looks of things the Legion hasn't been around long, and our main three (Cos, Saturn Girl, and Lighting Lad) are still very wet behind the ears and not great with their powers yet.
Also, if you go ahead and look at the second cover for issue 2 you can see the main three form the Legion looking just like they do now.