The University of Oxford says, "Plagiarism is presenting someone else’s work or ideas as your own, with or without their consent, by incorporating it into your work without full acknowledgement."

But yes, I see there is a debate about whether re-using your own work can be considered self-plagiarism. https://www.turnitin.com/blog/is-recycling-your-own-work-plagiarism

Fair enough - but I'd like to stress that I haven't sold copyright to a publisher, I have no obligation to a publisher, and the Medium publication offered the piece does not demand work has never been published before.

Selling second rights (reprints) of your work is a legitimate way for writers to generate a second income when they own the copyright.

I'm not breaking any laws. I'm not stealing someone else's work. I own the copyright.

I don't believe Medium prohibits writers from publishing reprints - or reusing sentences, either.