I think the idea was that with Harry the requirements of the ritual were fulfilled, though accidentally. One of those requirements is the death of an innocent.

But the HP wiki says that there's some kind of incantation that goes along with it, so that's either optional or... whatever. It seems to be like the Goblet of Fire portkey. The rule is the rule except when it isn't.

The biggest difference between Harry-as-horcrux and Quirrel-as-horcrux is that Voldemort doesn't seem to have killed anyone (as far as we know) to possess Quirrel. So even if Harry might have accidentally become a horcrux, Quirrel didn't, although he might have served the same purpose a horcrux does in "keeping the soul anchored to the mortal world."

I'm definitely not trying to argue that these things are consistent here, though. The point is that when people say something is "effectively" something else, they mean "practically" or "almost" rather than "actually." Unless someone finds some corpus data that suggests that Rowling's dialect (or, hell, her ideolect might be workable since she HAS written several rather large books) has a different usage...