It seems clear Fury underestimated Ngannou, he did not expect that sort of ferocious and fearless attitude in the ring. This was supposed to an easy win for Fury but Ngannou had other ideas. Fury stated he trained for twelve rounds and trained double the period he normally would for a heavyweight fight but we do not know.
One thing to note was Usyk today stating that Fury underestimated Ngannou but he will not make the same mistake when/if he meets Usyk for the unification fight.
Probably Fury didn't trained hard enough and he should now what is the consequences of it. So I do not believed it, maybe this is just an excuse on his part because he was almost beaten, or for some he lost to a supposedly MMA fighter with no boxing skills.