A browser using your keychain seems like the least questionable bit, if anything.
I really doubt OpenAI consciously wants my passwords, but I could absolutely see a poorly-coded (or vibe-coded, lol) OpenAI process somehow getting my keychain into their training set anyway, and then somebody being able to ask Chat-GPT 6, "hey, what's Analemma_'s gmail password?" and it happily supplying it. The dismal state of LLM scraper behavior and its support (or lack thereof) of adherence to best practices lends credibility to this.