I thought full stack means going all the way down to tuning the database, the kernel, building your own CI/CD, and performing your own deployment and scaling.
In practice, I've seen it used to describe someone who is fluent on the backend (DBs, APIs, DevOps, etc.) and competent on the frontend (React). But that could be more of a product of the talent pool, I think there are more backend engineers who dabble in the frontend (maybe because that's where the perceived party is?) than the other way around.