Just a quick eyeball, but DT Mini specs look like they're well over 62" too. Even the Brompton specs at ~57" can barely make the 62" airline requirement when you consider the additional space taken up by the case and padding.

Have you considered partial disassembly of your FS9 like the removing wheels, pedals, tire deflation, etc.? I have a 20" Dahon Speed P8 that can fit into a standard 62" Samsonite Oyster with partial disassembly and basically everything on quick release, except tire inflation.

I understand Southwest is super accommodating to the Brompton Mafia crew... they regularly take them as carry-ons.

https://bromptonmafia.com