There are rooting methods without tripping Knox and in those cases removing root will bring phone back to original 100%, so to speak. However once tripped, I'm not aware of any way to undo Knox flag. The method above does not mention anything about Knox and I'm 90% sure it does not remove Knox flag, it just removes the root using Chainfire method.
However, rooted phone shows up as custom right away in about menu and it's easy to see, where Knox flag only shows up in download menu and it's more involved to find, therefore the chances are, unless it is Samsung service, they may not look or even care about Knox flag as long as it doesn't show custom/rooted, especially that Knox is only Samsung thing.
I agree 100% "genius" customers are the reason for all the blocks and lockdowns on our phones. I just read some thread about some Einstein calling customer care asking them for help in rooting the phone, I'm kidding you not and he went on and on how useless the service department is when they tried to explain that they not support custom programing etc.