Luckily I did the math wrong on the 24 choose 12 (2.7M combinations).
That is simply the chance of choosing the correct 12 words.
You'd still have to find the correct order of them afterwards.
I believe this would add a multiplier of 12P12 (12! factorial)
So 24 choose 12 (2,704,156) times 12! (479,001,600) = around 1.3 Quadrillion (10^15) possibilities.
Hopefully that's correct.
I also missed that there is an "advanced" option that takes you through the process letter by letter. This doesn't compromise security at all. However, as described, there are so many other centralized attack vectors no one is paying attention to it hardly matters.