Hi there Jeff,
Sorry you got locked into the flat. Of course, sometimes things do happen, but of course, Airbnb has such a good App for instant messaging, that it is surprising that your Host did not reply. You can also call the Host directly and also contact Airbnb, who will try to call the Host for you.
Maybe the Host was unavailable at that precise time for some reason, which is not ideal, but it does sometimes happen - eg if I go to a Concert, I turn off my phone, or if I am on a flight, I am also obviously unable to be contacted.
As someone else said, if the doorknob was fixed, then was there any other problem? I suppose the Host doesn't know that you left before the end of your rental time.
If you didn't feel safe, because you thought that you might get locked in again, you could have raised a Safety issue with Airbnb, and they would ensure you got the refund, without the Host having any opportunity to deny it.
(By the way, it is not "illegal" for him to use his flat for Airbnb - it might be in breach of his lease terms, but that's not "illegal", but just a landlord / tenant contractual issue)