I thought "advertising agency" was referencing that the best minds of our generation are thinking about how to make people click ads at google/facebook.

I guess it was always a thing even back then?

I think he meant that the best minds of his generation were devising advertising campaigns and writing advertising copy for laundry soap and canned vegetables. Advertising was more text-centric then, and talented writers like Chandler could make more money writing ads than books.

Possible. But I think more likely is that it was a good frame of reference for him as he was an executive at an advertising agency for quite some time.

I don't think that's correct. Outside of writing, his career was spent in an oil company.