That person still appears to show as a Cat C rider as well
One of the ways to help stop the sandbaggers would be using Heart Rate into the calculation as well… If an event is set as a race then it should be set that heart rate is one thing that is an enforced requirement. Zwift generally has loads of data on your heart rate from all your rides so has a good idea of your max, avg etc. If not it always has the default 220 - age to roughly get a max number.
Using heart rate you would have a higher chance of picking up those riders flying under the radar or trying to play the system to Cat down or stay down.
If you see someone who is right at the limit of a Cat and their max heart rate or avg heart rate is well below % wise where you would expect it to be at effort then that should apply adjustment to get a more realistic number to use as your Cat Enforcement.
Also all these Zwift hosted Racing league races should always have a Cat E pen for those who do not have the data to properly grade them. Start in Cat E and prove where you belong to get yourself a ranking. These races are like every hour so there is no reason for anyone to be able to do any type of race series to not complete Cat E Zwift hosted racing to get yourself ranked properly before you can then join other races.