I'm not him, but it was pretty obvious that the comments section was going to be attracting more and more people saying the same thing that had already been said before, and that no useful discussion was going to be had. At some point the value of spamming everyone who commented on the issue with a notification (which puts an email in your inbox if you haven't changed the default setting) becomes lower and lower.

I've seen that before on other issue comment threads. The repo owner says "Hey everyone, if you want an issue fixed, please upvote the issue with a thumbs up". And many people don't read that, and instead post "Please fix this" comments without giving a thumbs-up to the issue. So, 1) the repo owner doesn't get to use the "sort issues by # of thumbs-up reactions" to see the priority of that issue, and 2) everyone who has subscribed to the issue gets spammed with a message that's useless to them.

Since nearly all the new comments had become "me too"-style comments, which should have just been a thumbs-up on a previous comment in order to reduce spam, I feel like locking the issue thread was the right move at that point, to stop people from receiving yet more unnecessary email in their already-overflowing inboxes.

I didn't. I have locked the comments on a closed PR, and many of those comments were not constructive.