For sure, the data he quotes is from Ruhlen's book.
Greenberg was, IMO, one of the greatest scientists of the last century.
but also admits that the evidence is pretty much all in--the world's modern and historic languages have been poked into the computer, the computer has crunched--and the picture is still ambiguous.
Ruhlen also says that the 27 etymologies he presents are the tip of the iceberg. I am personally optimistic that enough will be done to demonstrate monogenesis, but I don't expect to reconstruct the pronouns and declensions and so forth of "Proto-World". Maybe the phonology.
Actually, it's kinda like tracing multicellular life back to unicellulars - because lateral gene transfer is common among single-cells, there's really no tree when you go back that far. It's quite possible that there never was a single ur-speak, just a lot languages and dialects and multilingualism. But like life, there would be a single tree after that.
Part of the reason I'm optimistic is the bottleneck in human population something like 70K years ago. If they've already found cognates between Khoisan and Australian, and the Aboriginals have been there at least 40K years, how long ago was a language that contained the ancestors of the cognates spoken? Seems like we're more than 1/2 way there.
I guess I could have clicked on the link and looked.
I was pretty impressed with Ruhlen's book, but one of the criticisms of his method troubled me. It is charged that his samples across language families stretch the semantics, using here a word for "mouth" and there a word for "talk" and elsewhere a word for "tongue," etc. The lack of rigor in his method allows him to paint a misleadingly convincing picture.
However, I do suspect that language was invented only once. It's as least as easy as thinking that it happened independently in some N locations.