I disagree so hard with the notion that isolation is somehow done for you, that I don't even know how to begin arguing.

I'll admit to starting this confusion by reading what I thought would be written, not what was actually there. TFA does phrase it to my satisfaction that the test is isolated. I assumed when reading it that Kent thought he had isolation but did not. By analogies: a naive programmer does not perceive the existence of race conditions; a naive web app developer doesn't perceive the existence of CAP; a naive backend engineer thinks they have full ACID guarantees when they're only guaranteed READ_COMMITTED. I thought I was reading another writer just wishing isolation into existence through ignorance.

(So I accidentally straw-manned Kent's argument, but maybe that's not such a problem, because it seems you're OK with the straw man version?)

Anyway, maybe a question moves things forward.

JUnit's been around for decades. Every developer has had multicore dev machines for decades. Why would parallel execution not be the default, or a simple toggle that everyone would turn on immediately?