That's the first confirmation I see of it too, and I followed that pretty closely a while back. We knew they had both at x86-64 and a ARMv8 project running at the same time (K12 was the ARMv8) but it never was really spelled out how they implemented it (best I got back then was that they shared a lot of commonalities).
They cancelled it though as they saw : 1) the ARM server market wasn't happenning, at least not at the timeframe the product was to be ready for 2) the x86-64 could really be competitive with Intel
Then Intel had some issues and here we are.
So, it's finally confirmed! Zen had an ARM front-end after all.