If you do a dump like this all of it should be formalized, there's simply too much material to review by hand and additionally it is AI-written which makes it hard to read compared to human work.
With 40% formalized they probably have a good idea of how many were found to have fatal issues in the formalization attempt, and they hired some mathematicians to verify some of them, especially the big headline ones.
Are they all too busy having brilliant ideas? Doubt.
OpenAI math paper dump should be considered like a hint from 200 IQ eccentric genius - unreliable but perhaps insightful. If it was not "ugh AI" people would be happy about it.
It would be ridiculous for anyone to say "hey man you really shouldn't even have posted these unless you have an ironclad proof".