[peace4all]

He has written a big book on evolution of Earth and theories around it. [1]

It includes a whole chapter on plate tectonics, the preceding theories, and how plate tectonics itself was gradually accepted. In the book, there's also a large review on other scientific problems, like where the Sun gets energy from, why the Earth has magnetic field, where does internal the heat of the Earth come from, etc. Lots of "how do we know that..." questions. A great review, highly recommend.

So he's 100% aware what status plate tectonics had in Tolkien's time. I think he also mentions that as a linguist he wasn't a geologist, and could draw the land whichever way he imagined.

[1] Kirill Yeskov. History of Earth and Life On It. (in Russian) https://readli.net/istoriya-zemli-i-zhizni-na-ney/

[CrazyStat]

If he was 100% aware of the status plate tectonics had at the time Tolkien was writing them he should not have criticized him as he did.

[peace4all]

Maybe he was just sarcastic?

It’s a no brainer that a person cannot be equally oriented both in linguistics and in geology. ... So it would be quite possible to declare an amnesty to the Professor at this place: they say, an offense undoubtedly took place, but it does not pose a particular public danger.

[CrazyStat]

I find it hard to read the extended discussion of plate tectonics as sarcasm. It's not a single throwaway sentence, it's multiple paragraphs.