I would agree with the idea that the other person is presenting. I agree that in technical terms nobody is going to be able to say that there ever was a first human, but rather a population came to be first, then the term "human" was applied to it.
But if we did define people as individuals with specific genes or a specific group of genes, then at some point in time, one Individual animal would acquire that group of genes before it reached a general population. And that individual could be said to be the first true human. Regardless of what that combination of genes might be.
If we believe that humans exist now but didn't exist in the past, that beginning had to start somewhere. And if we say that this species is defined by X group of genes, then it follows that X group of genes wouldn't necessarily mutate in 100 or 1000 individuals all at once, but rather would come to be at a particular point in time in which a mutation occurred in an individual.