You are right and that was a bad choice from my side so I apologize for that.

The way I see this warning is that the "unexpected" is more related to the way we use to exclude rows when using this type of condition in the ON sub-clause on the SELECT statement. The behavior in the MERGE statement is different and these extra conditions are not used to exclude rows from any side, applying the conditions to all rows in both sides depending on the WHEN parts used in the statement.