Whose definition is this "newness"? Plagiarism is about passing other people's work off as your own. It comes from a Greek root about kidnapping, not about how old something is.
Your observation about the root of the word might be accurate, but (in my personal view unfortunately) is not relevant. "Plagiarism" is now used to describe previously published (and in some cases unpublished!) work that is being "recycled", regardless of author, without explicitly noting it to be such.
I'm not defending the usage, I'm simply explaining how the term is now used. Language mutates, origins are lost, and usage evolves.
You may choose to quote dictionary definitions, but dictionaries document usage, and of necessity can never be fully up-to-date with more recent changes. This is especially true when the terms are being interpreted within a specific context. In many academic environments "self-plagiarism" is an adaptation of the original concept to indicate that students cannot repeatedly submit the same work over and over again. They will say something like "Work for this accreditation must be new and original" and then use the term "self-plagiarism" to describe when that stipulation is breached.
As I say, language evolves. People even say "The proof is in the pudding" and "I could care less". These are idioms that convey meaning as an entirety and are not to be interpreted according to the meaning of the words therein.
Similarly, now in some contexts "self-plagiarism" is a term used to describe recycling one's own work. Sometimes it's fine, sometimes it's not.
Love it, like it, or loathe it, the term exists and describes "A Thing(tm)".
This is why I'm asking whose definition it is. Whose idea was it to start using this term, which had a useful and important definition for generations, to refer to something else now, and then act as though everyone should just know this?
I'd assume it would have had to be somebody pretty important, for their fiat to have such weight. I'd like to know who it was.
I'm sure you can use on-line search tools to find where and when the term first appeared on the internet. My suspicion is that the phenomenon was being discussed informally between people who assess students' work, someone used the term "plagiarism", someone will have pointed out that it was "plagiarising" the student's own work, so the prefix was added, and stuck.
It's a concept that needs a name, and when a name turns up it often spreads quickly with no obvious source.
That includes citing your work that is unpublished (there are valid ways to cite unpublished work), and building on it which would be fine in many cases.
You can quote any previous work, others and/or your own, but you must highlight/identify it as such.