Yeah, if it's unlockable in a reasonable amount of time (say, 5-10 hours per hero), then it's basically the same as those stupid F2P mobile games where you can pay to "speed up time" or whatever. Or if there's a rapid churn where you lose access to heroes after some amount of time (i.e. even if you pay, they'd disappear).

But if you can grind for a few hours to get the hero you want, then yeah, not P2W.