No, it doesn't make sense. You should be able to separate peace that target, and the war leader should be able to force you to give it back up if you can't get the war score for a white peace or something.
When you "guarantee" someone, that means you actually protect them, not hiding 100's to 1000's of miles away and expecting that your mere word will protect them magically. It's beyond comprehension that you think it's OK to manipulate war score without contributing.
Mostly this works against player's favor, because it's usually humans who take on the aggressive role...
But I was actually saved twice by this system where the warleader changes and the target of the war - which was me - couldn't sign separate peace. I didn't know it could happen to human players, but it did as the command went over to Zhou when blobbing Aceh and Hindustan almost simultaneously attacked my then-3PM Sukhothai. There are probably better ways to handle this, but as an in-game mechanism to give small nations some degree of protection, I'm quite okay with it. Zhou peaced out Aceh with their money, and surredered my provinces and their money to Hindustan, which I was not bitter about.
I don't get the point the AI doesn't try to intervene and actually fight for that OPM. It may take time for them to arrive, but I haven't seen the AI allies literally doing nothing and not even trying unless there's a real hindrance to getting to the battlefield, which is certainly not frequent.