The idea with the Trace Color A,B,C,and D from Red Sea is that they are replacing all trace elements, or the trace elements that or most likely be to consumed, not just a specific trace element. If you buy into their marketing they have done studies to understand who quickly each trace element is consumed in relation to calcium. The idea is that this are consumed at a fix rate and ratio. So dosing all 4 relative to the amount of CA you use will maintain the levels of all trace elements appropriately (at least according to Red Sea).
While you are correct that high end salt mixes are supplemented with trace elements, where you are slightly off is that all small mixes are actually composed of major, minor, and trace elements. The ratios may change from brand to brand but any quality salt mix should have trace element as part of its make up.
That brings us to the next point, why dose trace elements at all. Well it is basic chemistry and math. Or corals use element, major, minor, and trace, to grow. As the corals grow the level of each of those elements goes down in the sea water around them. If allowed to continue to deplete without replenishing the availability of the elements will eventually become rate limiting to grow. Now the question becomes, can’t I just do water changes to replenish my trace elements? The true answer is kinda, but not to the original starting level unless you do a 100% water change or you do a water change with sea water that has elevated elemental levels above what you started with in the first place. And this is where the math comes in... let’s say you have 100 gallons for water with 10 molecules of X element per gallon (so 1000 Xs in your tank). Over the course of some period of time 20 of those Xs are consumed. Now you have 980 Xs per 100 gallons or 9.8 X/gal. Now let’s say you do a 20% water change with new water of the exact same make up as the original was 10 X/gal. You remove 20 gallons of water that is at 9.8 X/gal and still have 80 gallons at 9.8 X/gal (784 total Xs). You now add the fresh 20 gallons (200 Xs total in that). I’m that 100 gallons you now have 984 total Xs or 9.84 X/gallon. As you can see there is very minimal impact on the level of the depleted element. While this example is extreme the goal is just to demonstrate the benefit of also dosing trace element specifically and not only relying on water changes which will never bring you up to the original starting level unless you do a 100% water change.