The article hints at that, it basically means he didn't have any formal education in art, little outside influence from established artists, did not get any feedback on his work, did not publish anything on his own; I'm not sure if he ever gave any interviews or context after the works were discovered.
Three-year-old's drawing posted on refrigerator: outsider art or not?
It seems that "outsider art" only really makes sense as "work produced by someone not participating in a certain socio-economic structure that some people have erected in connection with their art-related interests", which is a remark that has no value.
Someone doing "outsider art" is an outsider to something, but that something is not art itself.
That socio-economic structure is not art itself. The "outsider art" designation doesn't say anything about the art itself, only the circumstances of one who made it; it is a form of ad hominem. There is no such thing as "outsider art" just like "logical proposition uttered by a known liar" is not actually a real category of logical propositions, like "true proposition".
Art requires no membership in any club, or conformity to any shibboleths. Because art is a basic human expression; it doesn't require education, or discussion or anything else.
"Insider art" is not any more valid than "outsider art", because art doesn't require any conformance to a code. It's not like "outsider residential wiring", where someone who is not a trained and licensed electrician can make a code-violating mess that is unsafe. Becoming part of some millieu in connection with doing art is entirely optional.
The literal interpretation of "outsider art" is nonsensical, like this: "You may have produced a natural smell out of your anus, but since you have no formal education in it, don't follow influences of notable farters, and don't receive feedback on your work from connoiseurs of flatulence, it is 'outsider fart'."
