The "flash sideways" are just made up by themselves, it's their imagination. Anything can happen there and nothing has to make sense there, that's awesome for the writers.
How else do you expect a man, who sat in a wheelchair for 4(!) years and just had spinal surgery a couple of hours ago to walk as freely as Locke does?
Really, don't get your brain worked up about the "flash sideways", it's pointless.