Hi Hugo,
Writing a question to demonstrate the equivalence of SQL expressions is very brave, so well done for that. Small point: in the explanation you say, "the only way to rewrite the query is to reverse the table order", but this code expresses the same SQL semantic I think:
SELECT
e.Name,
c.Name,
o.OrderDate
FROM dbo.Employees AS e
LEFT JOIN
(
dbo.Orders AS o
JOIN dbo.Customers AS c ON
c.CustomerID = o.CustomerID
) ON
e.EmployeeID = o.CustomerID
AND o.OrderDate > DATEADD(MONTH, -4, CURRENT_TIMESTAMP)
WHERE
e.Position = 'SalesRep'
Personally I find this layout easier to read than 'nested' ON clauses.
Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi