I agree. I have a hard time to say best athlete ever for anyone. Each sport has its own specialties. When I think pure athlete, I think of someone who can go do any sport at any time.If given the time, probably. But you could say that about most sports.
How many sports could LeBron or Steph have dominated with their respective size/strength and skill had they started at it from childhood? Think of all the American football players who could have killed it in gymnastics (just one thing) and they don't have an olympic sport at all.
I do think there's an argument against 3x3 and a lot of other sports of how widely it is played.
Can Michael jordon do the rings, can the top football player take Michael off the dribble, can Biles hit a 90 mph curve ball and so on and so on.