"It is all my fault!"
Do you agree that "all" as a predeterminer modifies "my fault"?
But if it is true, "fault" should be formed in the plural and considering meaning, I think "all" can be an adverb and a pronoun in the sentence, but not a predeterminer.
What do you think? Thank you so much as always and have a good day.
Whatever label you want to give it, all means completely.