Recommendation engines work on vast amounts of data they have on you and whatever made you speak about thing X was likely preceded by your internet activity which is not very unique as a precursor to speaking about X. In other words, if other people do Y on the internet and then end up doing stuff related to X, the recommendation engine will show you X just because you also did Y.

The other explanation is one of your contacts who were part of the conversation did things that either directly related to thing X, which you spoke about, or something the algorithm see other people do that relates to X, and you got shown ads based on your affiliation to this person.

I've also worked at FAANG and never seen proof to such claims anywhere in the code, and with the amount of people working there who care about these issues deeply I'd expect this to leak by now, if this happens but is siloed...