Well failed concept characters don't need to be developed/scrapped. And even then, it's easier to just explain Byleth's moveset as a combination of trying to distinctly not repeat concepts from existing FE characters and also represent the three main house lords in some way, as they're all popular and have distinct weapons in their own right. To that end, the only moves that would resemble a MH type move (at least as far as specials are concerned) are Failnaught (neutral bow special) and Aymr (Down axe special), and even then, it's unlikely Failnaught would be MH inspired given how simple its concept is. That just leaves Aymr, which isn't conceptually that far out there either, being similar to Kirby's side special. I really can't help but think that the rumor is just bogus even if it's not supposed to be MH is referencing.