In theory you can learn mind reading from some fantasy universe and check every Apple person. Or ask a crystal ball. Or use some other way to collect full information about our universe, check every rabbit hole, so to say, and then confidently confirm "there's no Apple backdoor here". "Here" meaning this plane of existence.

In practice yes.

EDIT: Forgot - the "refused to cooperate" and "they have disagreements" things even in daily wisdom don't change the probability of Apple having made backdoors. It's PR. You most likely won't learn it from the news if they do, in fact, cooperate.