There is not a single example about why this was "wrong" (e.g., filter equivalences to other join types) or a single acknowledgment of good reasons the admitted

There is not a single example about why this was "wrong" (e.g., filter equivalences to other join types) or a single acknowledgment of good reasons the admitted technical and architectural SME during your first-ever real-world experience might have for doing this. So I'm guessing this author has more than once incorrectly calculated a mean because he innered when he should have outered.