In my experience, having a solid understanding of the next level down in the stack -- the foundation you're building your startup on -- is really helpful. We built a PaaS, and knowing enough about Linux internals to be able to work out what would be easy and what would be hard meant that we could focus our efforts on high bang-for-buck features.

So I'd say that understanding LLMs to the level that you get to by training your own baby one would be a solid foundation for pretty much anything built on top of the "real" ones.