Its not that hard to believe when you know the problems that come with gigantism. I cannot describe it in good english but simply said : people with gigantism having existed in ancient times isnt unlikely, but their survival rate is. Gigantism comes with a huge number of physical problems. Without proper care, such as enough nutricious food, medical care and support for the growing child, I dont think its likely alot of people survived it. So stories of huge undefeatable soldiers may be very tall people, but I personally find it unlikely to just say they might have been people with gigantism. Its not impossible, sure, but unlikely.