Big companies can't hide this stuff forever, you can eventually figure out, it is just better business-wise for them to just state they read your emails for ads, it can cost them a lot if they don't, plus most people don't really care.

A few weeks ago a friend and I were talking exclusively through WhatsApp about a trip to London(in Portuguese with our very unusual jargon), I didn't do any search for anything related it was all within our conversation, a couple days later Facebook had a post on my timeline saying: "heading to London? Check what your friends did there".

I don't remember reading anywhere that WhatsApp reads your conversations for ads, but it was clear to me that's where they got the information, it was spooky at first.