If enough of these "ancient" genes turn up, your explanation is the first to fall.
I don't buy it, at least not on such slim evidence. The admixture hypothesis presumes that humans moving out of Africa bred with Asian hominids at least 2 million years removed from those in Africa. If so, the genetic imprint should be much more dramatic and evident ("what you need is 10 or 50 loci--one or two is not sufficient"). I can see interbreeding with Neanderthals, but not with Homo erecti based on this slight evidence.
It's also worth noting that "vanishingly rare" is not the same as nonexistent. That a haplotype appears in 53% of Chinese but only 0.5% of Africans (in a survey with 7.37% margin of error) does not necessarily signify as much as it may appear. To state this differently, there are tens of millions of Asians today descended in some part from a literal handful of Ghengis Khan's comrades - and their gene dissemination was much more recent and diluted in a far larger population.
In other words, all this means so far is that some Southeast Asian alpha male was descended from a "vanishingly rare" African...
One more point worth noting is that there are a number of small, very insular African clans. It may just (quite easily) be the case that when you survey 177 people out of a population of more than 800 million that you miss discovering the band with, say, an 80% abundance of this haplotype.
PS. One more point worth noting is that a haplotype dated to originate two million years ago (as in the second study) would be from before Homo erectus itself moved out of Africa..
PPS. One last point I'd like to note is that a small survey which turns up just one person with a given haplotype could just as easily miss that person. All of a sudden the 4-5 million people represented by that person have 'dropped' to zero, and "vanishingly rare" is now "nonexistent" - although it's not. And, with a 7% margin of error, your extrapolated 4-5 million might actually be more like 60 million.
Even 4-5 million if accurate establishes that the given haplotype has been floating around on the continent for quite a long while.