They seem to think that "homo sapien" is the singular form of "homo sapiens."
This is not true. "Homo habili" is not the singular of "homo habilis," either. I've never heard that, but it's about as stupid.
A final sibilant does not always denote a plural, especially if we're working in a language that isn't English. Also, since it's pretty clear that "sapiens" is the adjective, wouldn't you expect "homo" to be plural too, if "sapiens" were plural? "Homo" is obviously not plural. (The plural here is, grammatically, "homines sapientes," by the way, for those of you keeping score at home.)
I know I've said this before, but...just...don't use words you don't know. It's not really that hard.