I'm agreeing with you.
That's what I'm asking actually: If the crater resulted in a massive rise in magma up from the mantle big enough to to be detected by its local gravitational influence, but the (very thin) crust is moving extensively, then why can you assume the magma bubble (located under the crust, right?) is still co-located with the crater?
Maybe he means that the crust was made thinner (think inverted bowl) at that spot, and there will be upwelling of magma into that inverted bowl wherever it happens to be at any time. (?)
I couldn't find the spot I referred to earlier, but his site has a simple graphic of the continents dancing around on the globe:
http://www.enchantedlearning.com/subjects/dinosaurs/dinofossils/locations/Antarctica.shtml
Think of the Sudbury irruptive...it's not UNDER the crust, but penetrating it.