Doesn't exhaust back up in between strokes because of atmospheric pressure? I think that's what they are talking about.

Too big or too open of exhaust on lower RPM long stroke engines will make an engine have to "push" exhaust through the atmospheric pressure every cylce.

Whereas a system that doesn't allow the pipes to equalize as quickly or at all in between strokes will get in increase in power because the engine won't be fighting the atmospheric pressure.

My .02 on the thought process behind that statement.