I think it depends on the scale of apps you build for. One can deploy a simple backend API or web app to a PaaS like Heroku or Zeit, or even a PaaS-like environment managed by another team, without knowing much about distributed. I would consider someone that can do that + build a modern front end as full stack.
IMO the average CRUD web app doesn't have the scale to have distributed systems problems. Also there are managed service versions of everything these days even when one scales up.
Yeah. And the scales have to get pretty crazy before "rails on postgres with some in-memory caching" stops cutting it. Your average developer will never see it.
I believe a basic understanding is enough to be a Full Stack engineer. For example, it would be enough to understand that my horizontally scaling backend would imply stateless servers. It may not be necessary to understand how the clocks are synced across my clusters.
IMO the average CRUD web app doesn't have the scale to have distributed systems problems. Also there are managed service versions of everything these days even when one scales up.