It's a totally understandable mistake

A pronoun is a grammatical part of speech, so the closest definition of its terms is going to accidentally be overly broad when you try to test every word by it. If you went strictly by the layman's definition of a pronoun, it doesn't seem to exclude "jawn." However, you'll notice that almost every grammatical article you find about pronouns will at some point list all the pronouns. It's just hard to define the form, function, and intuited rules of a part of speech using the very same speech

But it's very clear that - functionally - "jawn" and "thingamajig" are not pronouns...

> The dog walked down the street > > It walked down the street > > Jawn walked down the street

Jawn doesn't work there

> He helped himself to some food > > Jawn helped jawnself to some food

Or here. It doesn't work because it's not a pronoun. Misusing it this way is one of those things that natives would instantly point to as proof that you're trying to sidle into Philly slang with no clue what you're doing.

Pronouns can behave in sentences completely replacing the totality of the noun phrase. Jawn cannot. Jawn needs an article (the/a), number, or adjective to precede it. Because it's a noun.

> Two dogs walked down the street > > Two jawns walked down the street

This applies everywhere it turns up

> Be careful with this hot pan > > Be careful with this hot thing > > Be careful with this hot jawn

Jawn is a normal everyday noun like pan and thing. The only thing that's remarkable about it is that it can work for every case of a noun.

> I met your wife yesterday > > I met your jawn yesterday > > Yeah, I've been to that bar > > Yeah, I've been to that jawn > > Pick me up a hoagie > > Pick me up one of them jawns

Normally things like the suits, popo, John Doe, people, thing, thingamajig, widget, anytown, the boonies, cooee, etc. are more specific in their non-specificity, because to be completely non-specific would theoretically ruin the whole point of communicating

Jawn is so fundamentally built on being completely non-specific that phrases like "one of them jawns" will be tossed in to make things even more confused... and yet, it still functions in communicative speech! HOW? I dunno. I don't make the jawns, I just use them