I had one world-class player tell me that he could tell how good a player was in 2 boards... Later, he revised that down to 1 board... Later, he revised that down to just watching them arrange their hand... Later, he said he could tell by just looking at them
I think I read a statistic once that 80% of ACBL players think they are above average bridge players.
Bridge is not like chess. There are no 6-year-old bridge prodigies making clever, critical q-bids in complex slam auctions. Bridge skill is definitely accumulated. But players accumulate skill at different rates and masterpoint accumulation does not directly relate to skill accumulation. Worse - ACBL MP accumulation has been skewed for marketing needs rather than by difficulty. I read recently that because Sectional tournaments were under attended, they were raising MP awards at sectional events.
Looking at masterpoints to determine expert status is like counting drinks to determine if a person is drunk. A person who has had a shot glass of beer is definitely not drunk and a person who has had 10 shots of vodka is certainly drunk. But anything in between is subject to what they drank, how fast they drank, how big they are, how used to liquor, etc. Same with using masterpoints to determine expert status.
Certainly, anybody with less than 10 masterpoints is not an expert and almost all grand life masters are expert. Anything in between is subject to more analysis.
And that does not even figure in that some players are expert in some phases of the game and not in others.