A frequent counting mistake is to treat Witr as the sixth prayer in the numerical rule.
The source explicitly excludes Witr from the count of six Fard prayers that drops Tartib.
At the same time, Witr is not irrelevant: the ordering relationship between Isha and Witr remains legally significant in its own place.
So:
• Witr affects ordering;
• Witr does not supply one of the six Fard slots.