You could make the proposition that sequential code is inherently asynchronous in modern operating systems, because the kernel inherently abstracts the handling of blocking/unblocking your process.
Yes and no. From a practical point of view considering sync code as async is useless as sync and async code have very different usability characteristics and it is useful to have different names for the two domains.
On the other hand, form a more abstract, theoretical level, it is important to know that there is almost always an event loop at some point deep in the stack and sync code is just sugar over async and you can always transform from one to the other.
Yes and no, the concept that is being communicated is that your program inherently is asynchronous how-ever the context switching/block/unblocking is abstracted away from you so you can focus on what is important to you - which is solving your problem. Once you communicate that concept then it makes it a lot easier for people to create new mental models that better reflects reality. Then they can structure their programs that can take advantage of it.
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.
When most people talk about async code it is with single threaded (hence the need not to block the event executor) applications (web-browsers, nodejs ect.). How-ever on the other end of the delirium when people talk about mutex's (locks) they're talking about multiple threadeds working in parallel which you inherently need to consider if the code you're calling is going to either direct or indirect acquire the same lock or like you pointed out calls a api that blocks. In the single threaded environment the only cognitive load you need to worry about is one or two yield points, how-ever in a parallel system your main concern is mutex's, race-conditions, or dead-locks.
