They're emphasizing a precise distinction between "concurrent" (the way it's structured) and "parallel" (the way it runs).
Concurrent programs have multiple right answers for "Which line of computation can make progress?" Sequential execution picks one step from one of them and runs it, then another, and so on, until everything is done. Whichever step is chosen from whichever computation, it's one step per moment in time; concurrency is only the ability to choose. Parallel execution of concurrent code picks steps from two or more computations and runs them at once.
Because of the GIL, Python on CPython has concurrency but limited parallelism.
Concurrent programs have multiple right answers for "Which line of computation can make progress?" Sequential execution picks one step from one of them and runs it, then another, and so on, until everything is done. Whichever step is chosen from whichever computation, it's one step per moment in time; concurrency is only the ability to choose. Parallel execution of concurrent code picks steps from two or more computations and runs them at once.
Because of the GIL, Python on CPython has concurrency but limited parallelism.