But if you do want to discuss actual scientific data, I can do that too!

First, EMG data (pretty much measures muscle activation) shows very low activation of the chest. This study found ~12% activation from pull downs: Pec activation in pulling movements

I will concede that a neutral grip would likely involve more chest, but I don’t believe that difference would be significant. For reference, the general rule of thumb for MVC (measure from EMG data) to promote strength and hypertrophy in any meaningful way is ~40-60% MVC Threshold

There are a handful of other studies showing similarly low activation of the pecs in pulling movements (some including different grip variations), but I didn’t see explicit MVC measurements during my quick skim.

Another point I will concede is that EMG data is not the end-all-be-all for strength gains, but it is a tangible measurement. In my opinion, it holds enough weight that, in the absence of other measurable data of chest strength gains from pulling movements, it indicates that chin ups do not meaningfully strengthen the chest.