Still better than Katakuri's flashback
Katakuri's flashback gets a pass because he wasn't the main villain of the WCI arc and he wasn't a character that was build up.
Even if Katakuri's flashback wasn't shown, most wouldn't really care or question it.
Kaido on the other hand was not only the main villain of one of the longest arc in One Piece, but his character was build up ever since Punk Hazard.
He's a character that everyone thought would get a really good flashback especially after Big Mom's flashback back during the WCI arc, but unfortunately Oda decided to rush Kaido's flashback just because he wanted to quickly end the Wano arc.
I think Oda did a great job showcasing Kaido in terms of power level but I'm sorry, when it comes to his flashback Oda really failed on that one.