The subcircuit allows you to keep The A-line and associated .model line together, copes with the code model syntax and provides the proper pin assignment.
But this then always requires an external library. What I did for Eagle is that base definitions already came with a library for all of the available Xspice models.
Still interested to learn what is then the purpose of Built-in Xspice model, if you maybe know?
Code models require the instance line (the Axx … line) and a associated .model line. How to add that without an (external) library?
This external library has to be done once, then saved in a dedicated library to become an internal library. A role model for this is available in the Simulation_SPICE library: There is Simulation_SPICE.kicad_sym with all its symbols, but there is also a Simulation_SPICE.sp which contains models for the generic opamp symbol available in Simulation_SPICE.kicad_sym.