CSS "pros" (who masterise all concepts of the game) are still the best on csgo while best csgo players when they play CSS look average+ no more, i think that says it all
We have already see best csgo players being so slow on pipes on mako on CSS for exemple
They can't bhop properly, most don't even know how to run fast they just stay at 250 speed
+ If you are a top CSS player, when you play csgo you really feel it like a kid game not serious (yes we all started this game kids but many stayed because it looks more serious than that), in csgo aim shooting is easier, if you are a "pro" player, as a zombie in csgo you will not have an important impact, but on CSS a pro bhopper zombie can almost win alone on many maps
Also, you can be a very good ZE player on csgo with "only" 2k hours while on CSS you will probably still look complete average