I have a similar intuition to yours. On many topics it isn't so much self-censorship, it is simply that for many people there is rightly no motivation to go around announcing their nuanced, moderate, or no-opinion-but-unimpressed-by-extremes positions.

Even when people do have strong opinions on a topic (and a moderate opinion can also be strong), most people have better things to do with their lives than to go around blasting their opinions to the world as a hobby. And the few in this camp that do are not very likely to be amplified by the engagement algorithms.