I may suppose that this is a result of several less fortunate iterations (bulldozer, anyone?) which led finally to a machine that checks all interesting boxes.
You can't build a new thing right from scratch, but you can market intermediate imperfect results to cover the losses somehow; I suppose that's what AMD did a few years ago.
Now they have finally hit the 10, and reap the benefits.
That's my sense of it too. A ~decade of tinkering with this modular architectural approach is effectively an evolutionary process, plus perhaps valuable input from a once-a-generation expert like Keller, results in leapfrogging the current state of the art.
The key, as you say, was surviving an unpredictable amount of time till the tinkering paid off.
Zen Is really a from a scratch design far from a refinement of bulldozer which was a dead end. Instead of experimenting with exotic architectural solutions they built a unexciting but extremely solid design (i.e. the improvements and wins were in the small details, not the overall architecture) and won. This parallels the years that Intel wasted on the dead ends of Netburts or Itanium.
Maybe I went too far back in time with Bulldozer, but Zen 2 is direct evolution of Zen. And to the extent that Bulldozer was an experimentation with a new multi-core architecture that more or less failed, I'm pretty sure AMD learned some valuable lessons that informed the Zen designs.
From what I read, AMD explicitly chose some details to be close to Intel's, e.g. cache size, size (and existence) of micro-op cache and so on. Software / compilers and Intel CPUs are optimized for each other, so it's best to be similar and a maybe a little better where it makes sense. While that strategly doesn't yield something vastly better than Intel has, AMD really needed something good so it wouldn't go bankrupt. Due to the process situation and a few nice energy-related innovations like clock stretching (allows running at voltages very close to instability), AMD actually did get more power-efficient high core count CPUs.
You can't build a new thing right from scratch, but you can market intermediate imperfect results to cover the losses somehow; I suppose that's what AMD did a few years ago.
Now they have finally hit the 10, and reap the benefits.