It works like this in the playoffs all the time. McDaniels was restricted by League rules about when he could sign and announce.

The team went ahead and presumably (dangerous I know but I bet that is what happened here) signed his preferred coaches to lock them up before they landed elsewhere because they were not restricted.

It was a handshake agreement without a signature, as his agent made clear when he dumped McDaniels. It wasn't the unofficial part that was weird, it was backing out after coming to terms, agreeing, and hiring coaches.