Actually, Freedom Passes can't, I've tried on several occasions to board non-TfL buses in the London area before 9:30 (84,402,477 and the 757) only to be told that I have to pay. I did some research and indeed, Freedom Passes can't be used on non TfL buses before the 9:30 time, even when boarding in the London boroughs.
I think that's changed since I last looked it up.
https://www.londoncouncils.gov.uk/services/freedom-pass/using-pass/bus
Perhaps it saves a little money without inconveniencing many people.
Your pass is also valid on local bus services run by other bus companies which are outside the TfL bus network. Your pass will be valid Monday-Friday 09:30 - 23:00 and anytime at weekends and on public holidays.