You realize that by "parallelism" the author is referring to the detection of concurrency issues, and not the parallel running of tests, right?

Typescript/JavaScript libraries are in the list, but JS it's single threaded, so I don't know if there is really something missing here.

JS is single threaded, but the process lives in an event loop.

You can easily write a JS function that can have a race condition by launching two asynchronous processes (such as writing to local storage in a browser or the file system both operating on the same data).

I had this happen just today while scraping different websites writing to the same files and overwriting each other.

Concurrency is not the same as parallelism. I’ve understood TFA to mean concurrency when it mentions parallelism. This would explain the presence of JS libraries which could test for concurrency problems in asynchronous code.

What's stopping your test suite from running instantly?

Typically a desire to have tests that are somewhat realistic.

What does that mean?