Set and scalar should be regarded as data types in a more traditional programming language, and treating a 1-element set as a scalar is a type coercion that works as much as other type coercions: only if the data matches expectations.
you're missing the point: it's not about coercion or data types per se, it's whether the error happens at parsing or runtime; of course the software has ways to figure out intention but is it something we actually want?
it's not coincidence that the article complains about the same thing but in reverse:
> This is legal, but only if the subquery returns not more than one row. Otherwise, an error would be raised at run time.
except the 'fix' reintroduce it in a way that's subtler and way hard to detect because now everything is a set even when the intent is to have a scalar.
