I'm surprised that isn't blocked by cross-site scripting prevention or something along those lines. Shouldn't forcing logout require that you have the cookie that proves that you're logged in in the first place?
But yes in general making actions happen in response to a GET request is generally a bad idea, since these are often cached and considered "safe" to retry.
Logout (or anything else that triggers changes of any kind) shouldn't be a GET request.
For a large IDP like Google, Facebook, Microsoft, etc, you start to want* logout via cross-site requests, but it needs to be protected against DoS attacks like the link provided. So you need some other proof that the requesting site has a session - a session ID, an ID token, a unique identifier for one of the users in session (this is the least bulletproof one).
* well, standards wise at least. Google doesn't document a logout url anymore.