I disagree. Asynchronous means non-sequential programming, of which multithreading is one type. Or to be more specific, multithreading is a sub-set, or implementation, of Asynchronous programming.

Asynchronous programs can run on one thread or they can run on multiple threads. Asynchronous code ...

[...]it may appear like multithreading is hard[...]
[...]asynchronous code[...]

I stopped reading when you mentioned the two in the same paragraph. Asynchronicity is not the same as multithreading. It is important to distinguish between the two...

Your misunderstanding is extremely common ...