I'll explain.
1. He put on Grip
2. He changed it to Red Dot
3. He didn't change the first perk
4. Notice Perk 1 is red when he puts on Red Dot ( meaning that you have the ability to change it but he didn't)
5. He fooled you guys
Hope this helped you guys understand what he did.
EDIT: at the end it says edited by...
I think this one didnt work for the guy but there is another vid on youtube of a guy doing it with the saw and he has the red dot and it says for perk 1 weapon attachment