The way most people think about it, "client" is your local machine and "server" is the remote machine that has lots of applications and is potentially multi-user, but X turns that backwards. The big iron is the client and the relatively dumb terminal is the server.
This is because of its mainframe style history and technically it does make sense, it's just that everybody else does things the other way around.
For the people who weren't around in the ancient mainframe times who end up messing with Linux for the first time, this is confusing for a while.
Add to that that the user manages the ssh connection while the X connection is managed for them...
The server is usually a remote machine, especially back in the time when "client-server" architecture was emerging in mainstream (business) vernacular.
This has been true for decades.
Nevertheless, X11 "server" and "client" have confused very smart and highly technical people. I have had the entertainment of explaining it dozens of times, though rarely recently.
And honestly, still, a server is usually a remote machine in all common usage. When "the server's down", it is usually not a problem on your local machine.