we were taught for our atpl's that the definition of a spool is a compressor, a shaft, and a turbine combined. anything less is not a spool.
SL's reference is a rather narrow definition - valid for turbojets & turbofans.
But in the rest of the world a free turbine engine (i.e. separate power turbine, with only aerodynamic coupling to the core) is called twin spool, whether coaxial shafting or not.
(Were I of a combative bent, I'd point out that a propeller is really an unducted single-stage compressor rotor, and thus the free turbine+shaft+gearbox+prop of a PT6 constitute a "spool" per his definition.
But I won't press that point...
)