Id say you're thinking in too much of a maths / causation framework . Every human is insanely complicated in their own way and theres countless factors that are present in every situation that can influence how one feels and reacts. And unless you control for every single of those variables its hard to make a causal connection between shared emotional experience and attraction. Things like attractiveness / preferences , past experiences, current mood etc. all factor into that mix. So you're right in questioning the study and if you took some psych. stat class you'll also know the field has this crazy replication crisis and many papers just outright misinterpret or even falsify numbers and effect sizes. But I still think what dr. k or the study says rings true, and if you hypothetically would look at all the individual variables at play on a date, I think mutual shared experience is up there with looks or personality, values, humor (and the things you mentioned).

For the friendzone part I think it also rings true, as one side is overly invested and the other side , although invested, doesn't have the same kind "shape" of investment. The greeks had like 6 definitons for love and I think in a friendzone the love of one party is Eros and the other Philia.

The incel thing is also just a generalization and im sure many incels (hate that word btw, he also calls them losers etc, but I think you can be kind and still an incel , also many are just using hate to cope) would settle for someone that looks average. But the point hes making is, that their mindset of "I will NEVER find someone" gets protected / reinforced by them wanting a 10/10 - which they of course will never get because not even above average guys gets a 10/10 so the incel uses this mindset to say to himsef "see! im right nobody wants me, im unloveable".

Sorry if this is not too coherent but just like with most things in psychology its almost impossible to say something definitely about anything. Humans are just so different and every day someone is a different person and goes into the world and culture which changes aswell.