"Jim Macklin" wrote in message

seated, same for a 421C, same for a lot of airplanes that
are called "cabin class" by a sales department that is
trying to sell a charter.

The 400 series Cessna and several other similar craft have been defined as
cabin-class for years not only by sales departments, but by operators and
feds, too, each of which carries as much or more legitimacy than your cited
article, which is naught more than someone else's sales pitch. Nice try.
Stand-up headroom would be a valid standard to start with, except that you'd
be about 50 years late in applying it.