Right, workshop support isn't a reason why they cannot mod the client.
In trying to translate Grandpa into reality, I'm just guessing why a modder might say something like that is they worry about how to distribute and update a modlet to a server's player base. Right now that's manual and I know from my own experiences with distributing a modlet not all players can successfully install one manually without assistance. With workshop support it could be possible for a server to tell the client which modlets to install and for the client to download those modlets from the Workshop.
It might also become possible for the client to associate certain modlets with certain games so it could enable/disable them on a server-by-server, game-by-game basis, which would be convenient for players in more than one game.