Perhaps I'm missing some important aspect, but what would be the benefit of lying about this? How would serving video that didn't promote engagement help FB at all? Just more storage and bandwidth without increased opportunity to serve an ad -- backwards from how I understand FB's model.

People do say something false for a believed gain all the time. But usually when I hear something false it's a misunderstanding or misspeaking. So based on my (relatively naïve) model of how FB works as a business, "lying" doesn't seem like the right word here.