TheQahnaarin
Son of Skyrim
I think Mankar means the Dawn Era when he says Mythic age. He's not really an Elven patriot, letting Dagon rule the world isn't going to put elves back on top. The Thalmor most likely favour the Aedra (Auriel).
The Dawn Era and Merethic Era share certain events, that make them similar. Though it is entirely possible he meant Dawn Era, he does make several mistakes in his dialogue. I'll just say he's insane.
I read a bit more about this, and White-Gold is special because the city is shaped like the Wheel, but there also seems to be a lot of evidence that its deactivated. It's probable stones (either the Amulet of Kings or a line of Dragonborn Emporers) are destroyed. Also on Alduin's Wall: "When the White Tower falls."
It still contains immense power, it was sought after by the fellow on Umbriel. The evidence it is deactivated is sketchy at best, since the tower falling can mean even for an instant. When Martin shattered the Amulet of Kings, it caused the tower to function without the needing of the stone. One could argue the great statue in the Temple of the One formed by the avatar of Akatosh, has become the stone or removed it's need.
Edit: I believe in the novels it is mentioned or hinted at, White-Gold Tower could create or be turned into a great weapon. Either way, we don't want the Thalmor getting their dirty elven hands on it again.
Difference is Mede's aren't Dragonborn. No Amulet of Kings, and probably a deactivated White-Gold Tower.
Mede's don't need to be Dragonborn, Martin sacrificed himself and shattered the Amulet to take on the form of Akatosh. Removes the need for a Dragonborn ruler, forever.
Not nessacarily insane, you do have to really to think about some of the stuff he says. Let's just agree he means a very early age, and if he does mean Merethic, it's not because he wants what the Thalmor want.
I regrettably haven't read the novels but I'm guessing going for the White-Gold Tower may also have something to do with overthrowing the Empire. There is hardly any info on the novels on UESP so I could be completely wrong.
Regardless, I don't see how holding on to Skyrim will help the Empire control the Tower in the short-term.
I mean there's nothing all that special about the Medes, and it doesn't need to be them. Better than not to be Dragonborn anyway.