Perhaps they mean sleeping across two facing seats, with their feet on the opposite one?
I've seen tall younger blokes with their feet not only on the edge or cushion of the facing seat, but they're half slouched down in the seat they are occupying, with their feet resting half way up the facing backrest. There's never a guard around when you want one, and sorry to say even when there is one they often don't call it out.
Came back from Fort William on a Scotrail to Glasgow on Friday, and a young girl across the aisle had her feet on the seat she was occupying, walking type boots resting on the edge and her knees under her chin, for almost the entire journey, except when she bent her legs under her for a short time before going back to yoga position A. It didn't even look comfortable, goodness knows why she couldn't just sit properly.