Well that's way too easy. I supposed I improperly assumed that the results would be cached longer than the TTL of the TXT record based on the expiration in the well-known text file. In that case they should have only seen a tiny window of time that emails remained in queues. I will have to read up on the RFC for MTA-STS a bit more.

In that case I would suggest they did not really shoot themselves in the feet. Rather they may have stubbed their toe.