Why would genetic mutation not account for it? We see it all the time with dwarfism and gigantism. Except in this case the mutation was most likely carried to several generations then died away.
I am speaking about present evidence. If you want to speculate and say that giant men could exist because of a mutation, well, yeah, of course.
Even nowadays we have the technology to alter our genetic material and IMPROVE it, but some "smart" people thought about moral dilemmas and such action is prohibited, not to mention that there are close 7 billions people on our planet and we don't have a solution for the rapid growth rate.
But again, if you want to speculate on genetic mutation, first you have to actually have proof that those giant people have an ancestor here on this planet. That's where you might have a problem.
I mean, with humans, we have a lot of ancestor links (there are missing links as well of course), but we can still trace back.
With the giant people trying to trace back might be a problem. Anyways, this is quite a recent discovery.
And in the history class I haven't learned anything about giant men in our past unless of course those were legends.
P.S.: If you are that curious, search Google for "giant men found in Bucegi mountains". We can talk after.
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