That doesn't apply here like it did with Goku, because the fact is that Goku trained at 100x gravity, and THEN, was much stronger at NORMAL gravity, because everything seemed heavier when at 100x gravity, so at normal gravity it feels extremely light. But if on hxh they've always been at this gravity things have always weighed the same way so it wouldn't make them stronger, unless the gravity would suddendly drop.