I saw that article (you'll probably notice a rather familiar reply in that discussion 😉 ) and although it points out an interesting technique using a CTE, it also completely misuses the logic of the ON clause coupled with "WHEN NOT MATCHED BY SOURCE", and then tries to fix it.

I just don't understand why MERGE seems to get so many warnings about how it can be misued, when I don't think it is any more or less dangerous than making a mistake when writing a DELETE which uses a correlated subquery.

The warnings kind of distract the reader from understanding the fairly intuitive logic (although admittedly cumbersome syntax!)