I'm glad to see that more people are working on making WordPress themes accessible. I would also like to point out that the Flagship Compass starter theme is and has been accessibility-ready since it was released and I'm aware of at least one site built on it that is 100% WCAG 2AA compliant.

Rian Rietveld, who is a member of the core accessibility team, did a full audit of our theme before we released it and also contributed some a11y enhancements to our mobile menu. I'm sure there are areas where it could be improved, but to be honest there hasn't been a ton of interest in a11y stuff from developers using it, so we don't really know where the weaknesses are.

If they really want to push accessibility out to more sites built on WordPress, I think Brad is right. Working with the underscores team to improve a11y will probably be much more effective than creating a new starter theme.