No, it wouldn't, because there are many modes of speciation which work just fine even with populations that are "always in contact".
Furthermore, this article (an editorial by the anti-evolution creationist group "Reasons.org") didn't even establish *that*. It just shows that 4.5 million years *after* the human and chimp lineages separated, at least one chimp ended up in a region where human ancestors have been found. Big whoop-de-doo.
It would have been harder for the two species to diverge genetically if they were constantly breeding as one population during this period.
Define "harder". Is it "harder" to have a mutation which triggers sympatric speciation, or is it "harder" to become geographically isolated? Both seem not "hard" at all -- populations keep living their lives, and thanks to nature s**t happens (without any effort at all) that has an effect on the future of your descendants.
It either happened or it didn't, and mountains of evidence overwhelmingly indicate that it did. In any case, again, the creationist spin in no way establishes that this *was* even the case. They're just propagandizing by misrepresenting the implications of a find.
Whether they do this through incompetence, or dishonesty, or a combination of the two, is left as an exercise for the reader. Lord knows the creationists have exhibited an endless capacity for both.
Actually, if you read the Nature article, all they know for sure is that three chimp teeth ended up in Kenya. Creationists, of course, ridicule inferences from fragmentary fossil evidence, except when they think they have something that disproves evolution.
My alternative scenario: a conversation between two Homo ergasters....
"Ug! Ug back! No see long time. Where Ug been?
"Been way out direction sun set. Land of big trees. Big trees, many trees."
"Ug find food good to eat?"
"Much food good to eat. Especially hairy tree men. Ug kill one, eat, save teeth. Look see!"
How would that work? If a population of critters is always more or less one group with no geographic borders separating them into sub-groups, how would speciation occur?
After all, wouldn't any mutations in the population spread throughout the population?
Now, the population might evolve, as a whole, from Species A to Species B over a long period. However, unless the population is separated into subgroups, I don't see how you could end up with two different species at the end of this period.