>>96324939
They have a webway portal on titan.
There's a webway portal under the throne.
They have psi-titans on titan, who are otherwise only found on terra, also following some secret order of the emperor that wasn't the mere defense of the palace, and there's also another body of (anti-)warp warriors tasked with the continued functioning of the golden throne who could have a similar order and would possibly join the grey knights in this task.


The thing is, the premise itself is flawed
>and somehow Terra isn't destroyed
the point of the decree is to keep the emperor fulfilling his duty in keeping terra safe and the astronomican running, they don't want to keep the emperor on the throne out of some kind of hatred, so if he leaves the throne it's given for granted that there will be problems, and that if he could leave it without problems, then the decree wouldn't be carried out.