Ok, I can see how this explanation is somehow consistent. The link you gave is just another observation of that, though. Can I infer from anywhere in the official docs, that this is correct and expected behaviour?
"If the expression is row-valued, then IS NULL is true when the row expression itself is null or when all the row's fields are null, while IS NOT NULL is true when the row expression itself is non-null and all the row's fields are non-null. Because of this behavior, IS NULL and IS NOT NULL do not always return inverse results for row-valued expressions; in particular, a row-valued expression that contains both null and non-null fields will return false for both tests. In some cases, it may be preferable to write rowIS DISTINCT FROM NULL or rowIS NOT DISTINCT FROM NULL, which will simply check whether the overall row value is null without any additional tests on the row fields."
