I read the paper. Let me cite it:
"That was the honor code of the time"
Unfortunately nowadays there is no such thing as a "honor code". I would not care what some blogger wrote (btw, the Quanta Magazine paper does not give a working link to what he wrote). Nowadays Internet bloggers cannot resist the temptation of using sensational titles. This has a simple explanation: a blogger's income directly depends on the number of clicks. So even some decent bloggers use this kind of titles.
But until now, Quanta Magazine had all my respect. On my opinion, it ruined its reputation by this article. Mainly by the title and abstract (the paper itself has some interesting contents). The title and abstract is the only thing which "general public" reads (see the comments to the question, for example).
Of course, it is interesting that Dedekind's letter was found and published. But the word "plagiarism" is completely out of place here. I cite the article again:
Cantor proved all these great theorems, but Dedekind was probably the greater mathematician.”
This is the opinion of most mathematicians. During their lifetime, Dedekind had much higher status than Cantor, they were friends, and corresponded, so Dedekind could be generous towards his younger colleague.
Cantor's reputation is NOT based on the theorem on the countability of the set of algebraic numbers, which Dedekind communicated to him. The revolution that he made begins with the discovery that a continuum is not countable, and this discovery is due to him, as far as we know. It matters little that Dedekind proposed a more elegant proof of this.
Another thing is that "general public" tends to promote a cult of the great, and to credit everything to them. Let me cite the article again:
Every branch of science needs a hero,” Ferreirós said. “Chemistry has Lavoisier, mechanics has Newton, relativity has Einstein. There’s always this one, only one. But that’s always a lie.
On the other hand "general public" likes the stories. of "plagiarism": Newton stole the law of gravitation from Hooke and the Newton-Leibniz formula from Gregory. Lobachevski stole the idea from Schweikart, Einstein from Poincare and Hilbert. And so on.
So we have yet another story of this kind.
Remark. I also read the paper of Jose Ferreiros, "On the relations between George Cantor and Richard Dedekind, Historia math. 20 (1993) 343-363, which is linked in the Quanta Magazine article. All facts were known in 1993, and it offers a much more balanced exposition of events. The only new thing which Quanta reports is the actual discovery of two letters of Dedekind, whose contents was reconstructed from other sources long ago anyway.