I know there is a max_fps option though I'm not sure if it's just meant for client-side.

Even DSL these days can handle more than 20k rates, so 10k is really only for those on dial up.
sv_minrate 10000
sv_maxrate 0

All FF servers should have their tic rate set to 66 and a good min/max allowable is:
sv_maxupdaterate 86
sv_minupdaterate 66

This means if a user has 5000 rate and 33 updaterate and they join a server with the above settings, it forces them to use 10000 rate and 66 updaterate.

I really mean it in terms of ticrate. Not having the server at the proper ticrate makes concs go wonky, like in the OT server.

33 is too low but was the default for FF for so long. I think the default is now 66 which is the highest you should set it and I've gotten O-T to change from 33 to 66 as evident by using net_graph. It's not their tic rate that is causing the problem. I want to say it's either a plugin issue or something else with their server config that is the problem.

Probably SV_Pure is turned on, or like you said maybe a server plugin.