I think it would need to be a very low-profile low-weight mechanical key switch and cap combo for it to feasible in this setup. Scissor switches seem more ideal for this sort of thing layout-wise. And I'm typing this from my GMMK with Glorious Panda switches and thicker XDA profile PBT keycaps.

Looks like the person is training in that video. I'd assume after some period of time, you could type as fast as on the regular two hander.