I know i fight a loosing battle but “atomic” is the wrong term in the context of kicad. Atomic implies something inseparable. Especially in computing.
A better term to use is “fully specified”. The symbol specifies everything needed to finish the design already inside the library. It at least points to a footprint. The footprint used is however typically a generic one shared with many other symbols (Standardized packages are quite common. ) In addition such a symbol might also contain BOM information like order numbers, house part number, …
The nearest kicad can get to anything considered atomic is by having both a symbol and footprint specialized to one part number. There are some rare cases where this indeed makes sense. The footprint and symbol are however still separate and the footprint can even be reassigned within the schematic.