I agree that the distinction between neologisms and overloading existing words is an important one (and your examples are good!) - but I think the ordinary understanding of "jargon" covers both.
If someone announces "I'm going to stop using jargon!", and goes on to say things like "steel man", "shut up and multiply", "dark arts", then most people will agree he failed. The list of LessWrong Jargon contains plenty of non-neologisms like that.
Neologisms are a bit more obvious, but even the distinction between somewhat rare words (like "neologism") and specialized jargon (like "overloading") is pretty fuzzy.
The list of LessWrong Jargon contains plenty of non-neologisms like that.
ADBOC?
When a group of people talk to each other a lot they develop terms that they can use in place of larger concepts. This makes it easier to talk to people inside the group, but then it's harder to talk about the same ideas with people outside the group. If we were smart enough to keep up fully independent vocabularies where we would always use the right words for the people we were talking to, this wouldn't be an issue. But instead we get in the habit of saying weird words, and then when we want to talk to people who don't know those words we either struggle to find words they know or waste a lot of time introducing words. Especially when the group jargon term offers only a minor advantage over the non-jargon phrasing I think this is a bad tradeoff if you also want to speak to people outside the group.
Recently I've been working on using as little jargon as possible. Pushing myself to speak conventionally, even when among people who would understand weird terms a little faster, can be frustrating, but I think I'm also getting better at it.
I also posted this on my blog