The GP post's point was that, logical as this would be, the IAU definition explicitly states that planets are "in the Solar System". So no, exoplanets are exoplanets, not planets. And Pluto isn't an example of either.

No, The IAU definition does not say that. You and others are confusing the definition of a planet with its application to planets in the solar system (and thus to Pluto) -- IAU Resolution 5A: Definition of a Planet in the Solar System.

The IAU of course does not assert that only planets in the solar system are planets ... that would be ridiculous, and the interpretation of the IAU's stance as being so ridiculous is ridiculous and then some.

https://science.nasa.gov/solar-system/planets/what-is-a-plan...

"The definition of a planet adopted by the IAU says a planet must do three things:

It must orbit a star (in our cosmic neighborhood, the Sun).

It must be big enough to have enough gravity to force it into a spherical shape.

It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun."

https://www.iau.org/IAU/Science/What-we-do/Pluto-and-the-Dev...

"More generally, a planet:

a. orbits its host star, just as the Earth and Jupiter orbit the Sun,

b. is large enough to be mostly round, and

c. must have an important influence on the orbital stability of the other objects in its neighbourhood."