Yep, not everyone registers but it seems that as you improve your Cat, so does the quality of fellow racers. For example, B grade gets far less cheaters and / or sandbaggers than D & C.
Generally speaking w/kg is power / weight which in your case is in that race is 189w / 72kg = 2.6w/kg but ZP bases your category on 95% of your 20min avg (which is 2.49w/kg for your only race recorded in ZP) . I’d suggest that you continue with D, and keep an eye on ZP results. When you start pushing that power or more consistently, you will start getting UPG (disqualified) and you will know its time to go to Cat C.