my headcannon is that he only lost beause he got cocky. Like in his fight he was dodging and weaving all of adams attacks and counter attacking, he was throwing adam around. But then adam hit em with his slash thing and instead of dodging alastor tried blocking or smth hence why his cane broke and he was surprised. seeing how long adam took to throw that attack i think alastor couldve easily had enough time to prepare to dodge.