It's much deeper than that. A full-stack "backend" developer also understands networking and how to fully use HTTP headers as they were designed (or maybe even stretch things a bit as necessary). They understand that even though their ORM tool makes it easy to individually fetch a dozen objects from one database table, the particular method they are writing right now, will get heavily called in production and they should configure the ORM tool (if possible) or hand roll the code (if necessary) for a custom query to fetch exactly what they need in one call.

In short, full-stack developers just "get it": network, database, ui-design, system calls, OS configuration, software construction, front-end, back-end, etc. There are trade offs everywhere: risk, complexity, effort, cost. Usually solving improving conditions for one area of the system just push these trade offs to other areas. Full-stack developers get this and can help make better architectural and implementation decisions.