It's not odd, they explicitly reference the time and the closing of Fable. See the submitted post now as the link has changed to the founder's announcement.
z.ai posted an announcement earlier that day (in GMT+8) saying that they will make GLM-5.2 available later today at 5:21pm so it can't be a coincidence.
It’s just Occam’s razor since it specifically references “ Today, the sudden restriction of certain frontier models is deeply regrettable.” in the tweet.
5:21 comes twice a day, so they could have got it all ready if they wanted to. But I guess a lot can happen in 12 hours, and it could be a missed opportunity if Fable were re-released in that time.