The "maximum value" which camate refers to is the maximum SC damage for that final SC set after performing 6 weapon skills and achieving 5 SCs, not the multiplier. every SC set prior to the final one is calculated at its own SC multiplier term and inundation multiplier term.
The result is similar to magic damage where you calculate magic attack bonus in one term, then calculate magic burst in another term. you don't multiply magic attack with magic burst, you multiply the damage
Magic attack (SC multiplier which comes from the current level of WS element such as light or double light and amount of WSs preformed prior to this point), magic burst (Inundation multiplier)
automaton for example have their own inundation multipler (For their weapon skills) in a different term to rdm's inundation which comes from their SC attachment called speed loader. in which case if automaton closes the final WS of a 6 weapon type SC, they push the damage further.
WS dmg x SC multiplier x inundation multiplier x automaton's inundation multiplier = final damage (without including all other special multipliers such as SC trait, SC gear, weather bonus, resists, etc)