Embedded can mean different things to many people. The Wikipedia definition is:
An embedded system is a computer system with a dedicated function within a larger mechanical or electrical system, often with real-time computing constraints. ... Examples of properties of typically embedded computers when compared with general-purpose counterparts are low power consumption, small size, rugged operating ranges, and low per-unit cost.
The libreCMC project's definition is:
A device or system with very few resources and is designed to be run for long stretches of time without user intervention.
One of the main goals of the libreCMC project is to keep things really small in terms of operating footprint (storage and RAM used). Many of the devices that the project supports have as little as 4M of flash and 16M of RAM. In this scope, many SBCs (Single Board Computers) are well outside this range with some having more resources than an average desktop 10+ years ago.