As I understand it, the parallelism is about task execution and concurrency is about task structure. Or, as Rob Pike said:
"Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once."
He said that in his *Concurrency is not Parallelism" talk.
In the end I don't think it is too much of an issue. What confusion is really brought by conflating parallelism and concurrency? Sure, concurrent programs can be serialized onto a single core. But there isn't some deep conceptual unlock you get by having a strict conceptual boundary between concurrency and parallelism.
You can't unit test your way out, but if you care about the code's correctness, today there's a way.
Best examples are SQLite and Jepsen test suites for dbms engines.
Then comes the parallel debugging.
Pretty soon it's 15 years later, different person, yahoo-wee bro having long moved on.