You might be aware that on ZwiftPower you can see two different things, for other riders, which might be mistaken as race category limits.
There is a zFTP shown in watts which when divided by Zwift’s average weight system would give you a race category zFTP w/kg. As Paul has said you can’t know what their average weight has been over the past 90 days. ( you can probably take a rough guess)
There is also a 20 minute w/kg figure or watts figure which you can see. I think these are the 4.3 & 4.4w/kg in B cat races you can see. Two years + ago these figures would have been multiplied by 0.95 and if the resulting figure (of best 3 average) had been over 4.0 w/kg the rider would have been promoted to A cat.
Two years ago the ZP race category system was replaced by a Category Enforced (CE) system. For this system an estimate of their FTP/CP was made using various powers along a curve. More often than not this estimated figure, now called zFTP, didn’t match what people could see from riders’ 20 minute powers. At that time if your zFTP (still called CP at that time) in w/kg was over 4.0 w/kg you became an A category racer.
In August 2022 the category limits were raised for the reasons given in link below.
If effect, and I think believed by many, this just allowed bottom A cat racers to now become B cat racers. This may be one reason why you feel there are more very strong B cat racers than before.
But really confusing to many is why someone doing 4.3w/kg, 4.4 w/kg or even 4.5 w/kg are still in Cat B. Answer because the power curve calculation (originally brought in to cover less than 20 minute max efforts) is supposedly more accurate than 95% of an ACTUAL 20 minute max effort.