What it means is, if you're out hunting and you shoot a large unknown primate, during the season, and you have a license, you don't have to worry about the legal repercussions of bringing it home, taking it to the taxidermist, etc. It's a legal kill and you can't be prosecuted; though it's possible that the result of your kill will be the revocation of Bigfoot season and the creation of Bigfoot protection legislation, state-funded investigation, etc.
It also means that Bigfoot hunting licenses can provide revenue to the state, which I presume is the real reason for the law - promoting Bigfoot hunting licenses for novelty value could be a moderately lucrative endeavor.
If it's the season, and you don't have a license; or it's out of season, you shoot your primate and you take your chances with public opinion, but there will be poaching fines and social stigma. Although the amount of stigma against poachers varies somewhat from region to region - depending on how well-matched the hunting population's perception of balance in the local faunal assemblage is with the law's assumptions in applying bag limits and so on - it's usually a pretty strong disincentive, because native wildlife belong to the public and the hunting community understands that breaking the rules spoils things for all of them.
If you shoot a human in a bigfoot outfit, during the season, there will be an investigation, but in all probability law enforcement, the judge, and the public will be on your side, not that of the human who was dumb enough to dress up as a game animal during the season. If you shoot one during the off-season, that's manslaughter and/or poaching, depending on circumstances and the sympathies of the judge.
If you shoot a human during bigfoot season and claim that you sincerely thought you shot a bigfoot, which you have a license to kill, there will be an investigation, similar to those undertaken when someone accidentally shoots another human while hunting deer, raccoons, or whatever and claims that they sincerely thought they were shooting their intended prey. In that case, a lot will depend on how distressed you convince law enforcement, the judge, and the public you are about it, and the circumstances of the shot. One thing all good hunters understand is, that it's really easy to be convinced you see the animal you're looking for when it isn't there, and if what is there is another human being, or an out-of-season or otherwise protected animal, you done screwed up and better hope your aim was off. This is why established and posted hunting territories, bright orange hunting jackets, and so on exist, to reduce the chances of this happening. The results of such a shot can vary from getting off with a warning to murder charges, depending on circumstances and the state of pro- and anti-hunting sentiment in the area and in the officials handling your case.