Only one definition uses the word "pejorative," that is true. But several refer directly or indirectly to difficulty of understanding, slang, pidgin, and obscurity.
Maybe we just see the word differently. I consider it an informal word, not much less so than "lingo."
To me, it sounds like a playful nickname, not a proper term like "lexicon."
But there are many words that have different connotations in different places.
Edit: Here's another source, the more serious Columbia Encyclopedia:
- For Teachers