Yes of course you're correct. Any system of that kind is concurrent across its architecture. But the current context being considered is that of concurrency as a programming paradigm, which was established by the OP's observations on python's lack of concurrency which sparked the exchange. In the comment you quoted above the statement "none of the processes are concurrent" was meant to make the context clearer.
Concurrency is broader than just threads in a process. In your example, the whole problem is concurrent.