I just wanted to give an update. I'm hosting a CS:GO Surf Timer server @ 85 Tick on an Ubuntu 18.04 machine. The machine's processor is the Intel i7-4790K @ 4.0 GHz.
My server reached 46 players today and it appears the Linux limitation no longer exists in CS:GO (ensure the sv_parallel_sendsnapshot ConVar is set to 1). My server was using 140 - 145% CPU (an additional thread was being used) while the occlusion ConVars were disabled. With that said, tests in the original report have different results today (e.g. it reserves one thread when running threadpool_cycle_reserve today when occlusion is disabled instead of 0 when I initially tested on Linux).
In CS:S and other Source Engine games, having sv_parallel_sendsnapshot set to 1 can result in server crashes. However, I've never experienced this issue personally (even in CS:S). My CS:GO server has been running for 5 - 6 days straight with no server crashes while this ConVar is enabled. More information on the possible negative side effects of having this ConVar enabled can be found here:
ValveSoftware/Source-1-Games#11
I hope this helps and if anyone is still having issues, please feel free to reply. It just appears to be fixed from what I'm seeing.