Well there is video of his fights in MMA, at least the striking aspect is clearly shown there. So they could used it as a basis for them to gauge how good Ngannou will be as a boxer. For me they really underestimated him as he is a MMA fighter. And I do think that Fury is just saying post fight that Francis is good or even a difficult fight that Usyk (obviously he is down playing Usyk here).
Nevertheless, its a rude awakening for the champion and he is very lucky that the judges give him the win here. I know that there is a continuing debate as to who win this fight. But it's clear to me that Fury didn't prepared or trained very hard in this fight, he is just saying that he did after the beatings that he got so somewhat stroke his ego and not to hurt.
Doubt that this Ngannou mma career would be a good basis to evaluate his boxing. First of all octagon is much different from the ring. Ropes vs net, obtuse angle vs right angle, gloves difference. Difference in fighter movement is to much to take it as basis. As of you would compare ice hockey and bandy. Same stick+puck+skates, yet two different games.
However, I would agree more to you saying that Fury did not properly trained for the fight, than he underestimated his opponent or Ngannou was to good for him.
Ngannou has proven to give Fury a good fight. Let's say Fury didn't train well, but what Ngannou showed was more than enough to convince us that he will make a name in boxing. Give him another try; that was his debut in boxing. Now, I think his opponents won't make the same excuse about not training.
Actually, Ngannou and Fury are probably laughing at us since we still can't move on from the fight, while both boxers are already enjoying the million dollars they earned in that particular bout.