I would say that's a leaky abstraction. (The OS hiding the details of async by blocking your threads and context switching, although this is how many programs operate.)

The problem comes when you have a critical section where you need to hold a mutex (lock): If you're working with "proper" async code, you can safely call any non-async code from within the lock. (C# enforces this, because a lock statement can not contain the await keyword.)

When any method you call can block on IO, synchronous programming (IE, using the OS to become async), can leak and make you hold your mutex longer than you should.

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That being said, many of us build our careers programming threaded code that relies on the OS to block. So at this point we're splitting hairs.