Apparently, Mr. Seacat was asked at an L&L panel on Life in the IK about half-elves specifically.
He replied that humans and elves share some genetic similarities, but there would be very few opportunities for such a union to occur and the vast majority of any which did would likely be stillborn. As a result “there might be 3 of them.”
So there you go. It's possible. It still seems to be a divine typo rather than a genetic process, but he was not that specific.