> "Introvert" still doesn't mean "shy"

According to dictionary.com[0], that's exactly what it means.

Introvert - noun - A shy person.

[0] https://www.dictionary.com/browse/introvert

The APA's definition is probably more authoritative, and is inline with what OP suggested:

https://dictionary.apa.org/introversion

The goal of a dictionary is to be a reliable indicator of actual usage, so an authoritative dictionary is a bit of a contradiction. Almost all English linguists reject that sort of prescriptivism.

Meaning that the opinion of the experts is that a word's "true meaning" is however it is commonly understood by the people using it.

I think it's a bell curve and most of us are ambiverts:

An ambivert is moderately comfortable with groups and social interaction, but also relishes time alone, away from a crowd. In simpler words, an ambivert is a person whose behaviour changes according to the situation they are in. In face of authority or in presence of strangers, the person may be introverted. However, in the presence of family or close friends, the person may be highly energetic or extraverted.

https://en.wikipedia.org/wiki/Extraversion_and_introversion#...

I have quite a few extrovert traits but I am also shy and socially awkward. Mixing shy and introvert into one category leads to a lot of confusion.

Ambivert - noun - one whose personality type is intermediate between extrovert and introvert.[0]

I consider myself to be one.

[0] https://www.dictionary.com/browse/ambivert

I describe myself as an extrovert amongst mathematicians. (which, of course, means I'm still an introvert compared to most.)

At what point does an "introvert" become a "loner"? Or are all introverts loners by definition?

No. Absolutely not. They are totally different things.