I think they do allow transfers of less than $20k but in this promo, in order to get 30k FFD you need to send "20k or more". Its conveniently advertised as 1.5x FFD which it truly is if you send exactly $20k, so if someone had a legitimate need to send $50k USD at once, they'd still only get 30k FFD and in this instance I think plastiq would make $ on the transfer, whereas if everyone makes exactly $20k transfer, this would become a loss leader.

Now to get a 2X ffd on forex is probably going to be full of caveats, they're going to markup the exchange rate so it might not sound equitable judging it by the cover.