This is not a new formula it's held true for nearly every ship bar the Ktfdescgg fighter, so it's a product of inductive reasoning to say the arc with two cannons would have the same damage as the tie which it does.
And the HWK, Lambda, Decimator, Outrider, Falcon, VCX, Firespray, Fang Fighter, Upsilon, Mist Hunter...
Guns = k(firepower) was debunked waves upon waves ago and it never even had a basis to begin with. Attack dice are an abstraction of firepower, not formulaic based on counting the barrels on a model. Not all weapons are the same, and even similar weapons vary in performance.

