I don't know if Handel plagiarized or if Mozart "stole" a symphony from Michael Haydn as this article alleges and others have whispered.
But that's not what the article says. The only thing written about it in the article is this: "Mozart’s Symphony No. 37 is actually Michael Haydn’s Symphony No.25 in G Major with the addition of a slow introduction."
You read more into it and added the word plagiarize. Plagiarize means lying, pretending a work to be your own. But the article did not say that.
Works have been misattributed by editors and historians. That's a mistake of the editor. Plagiarism is entirely different.
It is unfortunate and a shame that someone like Michael Haydn does not get the credit he should for being a gifted composer and musician in his own right. Theoretically, capitalism (with its obsession over authorship and its system of copyrights and patents) should protect someone like Michael Haydn and the integrity of his "original work", but when the great Mozart comes along, we all bow down and submit.
Michael Haydn was huge in Salzburg, bigger than both Leopold and Wolfgang Mozart. He was already well-known when Mozart was six years old. Just because many people today don't know Michael Haydn does not mean he was unknown in his lifetime.
As for authorship, Haydn was a court composer. His work was performed in court fresh from the quill before it would ever be published later on. Hard to plagiarize from the leading composer in town when the work has already been performed.
The meaning of the quote “Good artists copy, great artists steal.” is that good artists imitate what they admire in others' work. But great artists steal – meaning they make it their own artistically. It's a quote about art, not money and profits from copyright.