So first, that means it's not a myth. It's a fact.

Second, things like physical resist make a huge difference on a giant. Several other races resist magical damage, but elves and arials don't - and you can't dodge a geyser or a pillar of lightning. This means they're more vulnerable to indirect (non-melee) damage AND not resistant to melee. Something shared only by felar and humans. Add into this that their hp is lower, and you've got a greater fear of invokers spells and similar mage mechanics than a dumb race.

Third, the difference is most noteable when they're equipped. Where an elf might be trying to stack on some extra hp gear, other races won't bother. Yet they still nearly always have that extra 150-200 hp on the elves and arials. Look at all the logs of smart races at their max hp. Look at all the logs of dumb races at their max. Ignore base hp and just look at how their stats and race effect their equipment decisions. Even if an elf or arial decides to focus on hp a bit more - they are always behind the dumber races. That extra hp has as much effect on their decisions to equip as it does on their final hp when it's all washed.

You seem to be trivialising 100hp, but the fact that the difference is always MORE than that combined with the inherent strengths/weaknesses of a class, means it is a much bigger advantage than you're making out. I'm not sure what you stand to gain by making this argument, but you are pushing the boundaries of fact by suggesting there's not a lot of difference. An extra 10-15% hp at hero is pretty damn sweet.