I'm not trying to say that they "should" have focused on Vulkan first, by the way. There may be good reasons to focus on OpenGL first. Maybe it's what Rosenzweig has the most experience with, or maybe getting a Vulkan implementation to the point where it can be used to usefully emulate OpenGL is more work than getting an OpenGL implementation to the point where it can do 2D and basic 3D acceleration, maybe Zinc has some serious performance issues or isn't solid yet, I don't know.
OpenGL is a much shorter path to stand up. The difference would be that they are already passing big chunks of GL conformance tests , but would take a long time to stand up Vulkan support.
This is about trying to get something daily drivable for most people in the most efficient way possible, and sometimes efficiency does mean implementing part of the tech stack twice if the first time unblocks you on other things.