It's a recumbent trike, this sort of design seems to still exist today, so presumably it works somehow.

https://www.rad-innovations.com/blog-our-news/about-recumben...

Just because they're niche popular doesn't mean they're a good idea. That said the position of the people on those trikes doesn't look nearly as bad as the C5.

I have occasionally seen continental European tourists on those in Ireland. They struck me as a really bad idea for another reason. They're very low to the ground, which is probably good for aerodynamics but terrible for visibility for people in trucks, busses etc. There's no way I would cycle one on any normal road.

Some people do have a small flag sticking up but I don't think that's enough.

Quite common to spot them in Germany or Netherlands.