I’m not sure why there is a “first speaker of French” fallacy. Does the existence of fine gradations means that a moment of transition cannot exist? For example, there is no clear moment where we can say that night becomes day. Yet night and day are distinct from each other. Thus, at some point, night had to become day, even if we can’t exactly pinpoint when. A similar situation arises when you look at a rainbow and try to distinguish at what point does the red become orange, or finding the point in history when Vulgar Latin evolves into Old French. In short, I do not think that fuzziness obliterates the possibility of distinctions at all.
The only alternative to this is if you bite the bullet and say that night and day are not really distinct, nor are red and orange, or vulgar Latin and Old French - they are just illusory distinctions. But this would seem to logically entail the collapse of all distinctions as illusory, which is to me is a much less philosophically appealing position to take. Even if you think it’s a defensible position, it’s certainly far from obvious that it’s the only plausible one, even taking into account modern science. In any case, I think this is a point where philosophers are allowed to weigh in.
One possible objection to the argument above is: how can there be a distinction if we can never pinpoint it, nor if we cannot scientifically test for it? My reply is that this presupposes a logical positivism where scientifically testable entities are the only things that exist. At most, you can say that a scientist as a professional scientist cannot say that there is a first speaker of French or that there was a first human. However, if you discard the assumption of logical positivism, why is a regular person not allowed to make that distinction, simply based on logical reasoning?
Of course, this doesn’t mean that we are allowed to conclude that in the speciation of modern humans, two humans must have arose simultaneously. That would be a miracle of sorts. But I think it is not indefensible to say that at some point, there was one hominid who was effectively the first modern human.