There are a lot of really strong riders on Zwift. Zwift is a global cycling community so you can expect to see the best of the best.
There are no doubt that there are People that weight dope and that is very sad. But there is no easy way to track that, Zwift Power (ZP) are trying to track weight changes and will DQ people that has sudden weight loss or irregular weight data.
Once Zwift auto assigned racing categories is based on performance weight doping will be less significant in the B,C and D groups.
Than mr Bob X that weigh 105kg IRL but in zwift weigh 70kg will be classified in the proper cat and he will have to ride his butt of to be competitive.
The only big issue will be A and A+ but they have started to ask for photo evidence of weight and duel power meters.