I agree that all designers should have some level of business competency, just like they should have some level of technical ability. The level is determined by their current position, interests, and ability. I think designers who are amazing coders are great, just as I think designers who are great in business are amazing.
I am happy to see a designer advocating that we should learn business. This viewpoint is under represented. Ever since I can remember I have aspired to combine art and business… and to some degree I found that aspiration as a product designer in a startup.
Every designer needs to find their own path.