If you have to 'search around' for your definition and make a dubious appeal to authority without citing any of those experts, perhaps you need to wonder if it's your definition that's wrong.
Further in the article he goes over exactly why he feels that its allowed to both test a single class or multiple at once, going as far to give a name to them: solitary vs sociable unit tests.
Then, he goes on to, and let me emphasize this, defend against the criticism that other people make that this type of test is not a unit test. (sound familiar?):
Indeed using sociable unit tests was one of the reasons we were criticized for our use of the term "unit testing". I think that the term "unit testing" is appropriate because these tests are tests of the behavior of a single unit. We write the tests assuming everything other than that unit is working correctly.
I'm not sure how thoroughly you're looking to be refuted here but I feel I can't quite do a better job.
Funny thing is, this is not even a very deep insight: the answer to nearly any question in software design can be boiled down to: it depends. This discussion is just the unit-test rendition of "it depends". Why are you so hell bent on having it exactly one way?
https://martinfowler.com/bliki/UnitTest.html
Martin Fowler
Although I start with the notion of the unit being a class, I often take a bunch of closely related classes and treat them as a single unit.
https://medium.com/@_ericelliott/i-use-the-well-known-defini...
Kent Beck
Unit tests test individual units (modules, functions, classes) in isolation from the rest of the program