There are a couple elements that a DDOS mitigation system needs to have.

It needs to be able to absorb the raw network traffic of the attack. A purely volumetric attack seeks to just overload the network pipes that lead to the servers. This can be with junk packets that don't even make sense to an OS kernel, but have a valid destination IP address so they get through the routers. If the DDOS mitigation system acts as a filter in front of the servers, it has to not get overloaded in the same way the routers do.

It needs to allow good traffic through to the servers. If the attack causes the pipes to just shut down and reject all traffic, then the attack has succeeded. So the mitigation system has to distinguish attack traffic from good traffic, and keep the pipes open enough to let the good traffic through.

For attacks trying to do expensive stuff on the database, or create spam posts, one useful reflex the system can have is to notice when an endpoint is doing those attacks, and then block it at the network layer.

That is not necessarily easy, and it requires control of the network ingress, which arbitrary hosting providers may not be able to provide.