That's not exactly how the books paint it. There is an actual webway portal IN the throne room. That's what Magnus broke through, that's what the Emperor was trying to open. The Golden Throne, at least as I understand it, wasn't part of that project.
I dunno then, from what I remember it was some ancient artifact that let him focus his powers into manually constructing a section of Webway to connect to the whole network or something.
Checking sources from teh internets, yeah it was a device that amplified psychic power that Emps MacGyvered into a preservation device when he got beaten up.