Like everything, it can help. But it's not necessary. Pretty sure all the eurobusts we've had over the decades played some soccer at some point in their life. And I say that as a eurobreh myself.
I guess this is another chapter to the good old "euro vs US fundamentals" debate. The difference is just at another level : kids in Europe play in professionnal leagues quite young where they have no choice but to walk the walk respect a rigid pecking order (there are some exceptions of course). Meaning they fall in line and do whatever they're told to. As opposed to US kids who play, well, with other US kids. And since the style of play is different (which is what Luka was talking about when he said that it was "easier" to shine in the NBA) isos just isn't something they end up doing much in Europe. There's much less money/star power anyway, even for veterans. Add all that up, it's harder to be a "star" in Euro basketball vs in the US, so the mindframe and the style of play of kids will be different. No amount of soccer or any other sport will change that, but the skills themselves that may come from another sport can of course be positive for some. Which is why I believe various players practice up boxing or whatever other sport in the offseason.
(And all that doesn't mean that europlayers are per se more team-orientated or whatever, but just that that's how they HAVE to play while in Europe. Doesnt mean they enjoy playing that way or even "naturally" inclined to do so. They grow watching the same NBA games and highlights as US kids.)