It does not scale quadratically per client though, and the number given is per client.

Let's say typical games send 1-2 megabits per-second for first person shooters with 100 players or less (in some cases it's more, some cases it's less, but let's assume this is at least reasonable to do in 2026)

Now you have a game with 1000 players.

That's 10 times the number of objects in the world (players have to be sent to other players, assuming you can see all the other players because they are right near you.)

Now this game sends 10-20megabits per-second.

It's just math. More players --> more bandwidth.