To appreciate his point, you must be aware that Harper is thinking about a formalization of concurrency. In such a formalization, you could have a deterministic execution of the concurrent processes as you hint, but you will also have a trace of incoming events in the formalization, not under the control of the CPU.
For the system as a whole to be deterministic, you would have it be deterministic for arbitrary event traces. This is rarely the case in practice though. Harper does not tend to just sling out a postulate unless he has good reason to think it is so, backed up by a formal system in which he identified the association.
It could be that he has identified concurrency goes hand in hand with non-determinism. To me, it does sound rather plausible.
It sounds poorly defined to me. Concurrency for me means a specific thing, and that specific thing does not have a necessary implication of non-determinism. My position is similar to dmbarbour's (https://existentialtype.wordpress.com/2011/03/17/parallelism...).
For the system as a whole to be deterministic, you would have it be deterministic for arbitrary event traces. This is rarely the case in practice though. Harper does not tend to just sling out a postulate unless he has good reason to think it is so, backed up by a formal system in which he identified the association.
It could be that he has identified concurrency goes hand in hand with non-determinism. To me, it does sound rather plausible.