The timeline looks about right, SA says he moved back to AMD in 2012[1], so figure 4 years to design a new core and get a few test runs in, then a year to ramp enough so you can sell said chip.
That being said, I think it has more to do with an overall culture change, the technical talent to build a good core has been at AMD this whole time. Jim Keller said as much back in 2014[2], and there is nothing special about Zen's architecture from a technical standpoint. What Zen is is a well executed conventional high performance CPU core.
By not splitting up resources like previous leadership at AMD, they were able to build a solid, competitive core. This type of investment takes 5+ years to come to fruition, from thought to test runs of silicon.
1 - https://semiaccurate.com/2012/08/01/apples-cpu-architect-jim...
2 - https://www.youtube.com/watch?v=SOTFE7sJY-Q