It's not nonsense - if you ride a fix, you'll find that it IS more tyring to have your legs carried round. The real source of the extra work out is slowing yourself by applying back pressure - more work, but also, more muscle groups.

For ILTB - a fix does have a very small advantage over a single speed with a coaster brake in terms of simplicity, and that's the lack of freewheel. I'm sure you can't believe that a machined cog threaded onto a solid hub is equally as complex as a freewheel. It isn't. In practise, I've never had a situation where that mechanical simplicity was an actual advantage, but that doesn't mean it's not simpler.