There's an old saying: you can never get enough of what doesn't fill your need.

For example, when you need sleep, you can't eat enough to make you not tired, but you may well pound a lot of caffeine and sugar.

If true, this would accomodate the simultaneous truths that:

(a) users accurately report their preference chronological friend connection when they come to a social feed

(b) users spend more time engaging with a social feed when the need they come to fill has irregularly payoffs

That you can get more engagement by not giving them what they want/need (or giving them what they need irregularly) wouldn't mean that they are lying to you, it would simply mean that engagement and social payoff curves aren't the same, and the incentives to drive one might not optimize the other.