If in a scenario where you can get twice figure cost by knocking them out (like in Stan Aside) you kill an archenemy, you get quadruple cost in victory points, right ?

Nope triple.

What is your source for that affirmation ?

He's basing it off of the rules for the Sentinel and Sinestro figures. If they capture and release members of their Arch-Enemy teams, then they receive 3 times normal value. I agree with the parallel.

This is very similar to the way modifiers are stacked up in the new D&D rules. Try to think of it as a "+1 figure point value" bonus instead of a "x2" bonus. Then you have 2 different "+1" bonuses added together, for a total of:

Figure value + KO bonus figure value + AE bonus figure value
= 3x figure value.