Zen1 was further behind in ST perf than Intel is today in it's desktop offerings. They really exploited their strength in MT and price, and showed that the market was already chafing under Intel's reluctance to go beyond 4 cores on their consumer line, presumably to avoid stepping on the toes of HEDT. But that just caused the competition to pretty much invalidate that entire line instead.

And I don't really see the situation being that obviously different if it was Nvidia who they merged with and Jensen was CEO.

The big issue was simply that AMD didn't have the cash at hand to both pay for ATI and maintain investment in R&D, at least without their next few products completely dominating the competition. I don't see a different CEO changing that. Unless Jensen was willing to value Nvidia significantly less than ATI at the time.