Remembering which guns are active and what their passives are is not nearly the same type of skill expression. Yes, there's twenty five different combinations of guns that Aphelios can have equipped at one time and that will require you to micro-manage the next one he equips in the main hand.
Invoker on the other hand requires you to memorize ten different reagent combinations and know exactly which one to use at any given moment in real time. With upwards of one hundred different combinations of abilities used possible ( Though you often times don't use that many ) that need to be used in rapid succession.
They are almost entirely different types of skill expression. Just like saying he's more difficult to play than Azir, Lee Sin, and Riven isn't entirely accurate. Especially since these are champions who have proven that you require _extremely intimate_ knowledge of their kits to play them at the highest level possible.