It's open source logic design, not semiconductor process. The SRAM for memory and caches is almost certainly fab-supplied too, as are the specific gate layouts, PLLs, etc... All that stuff is going to be secret sauce NDA'd IP specific to the process they were using (FWIW: does anyone know what/whose process that is?). Flash isn't fundamentally any different.

Presumably they just didn't want to devote engineering time to something nonessential for the first project.