You know, technically they
But seriously, if you completed 4 years of a theoretical physics Ph.D. but think your MBA was just as challenging... is there a reason why you were able to complete the MBA is less time (presumably)? It's because PhDs in Physics are harder (and possibly not as well compensated as a similar amount of non-Physics-PhD effort for someone intelligent enough to attempt a Physic PhD). Which isn't to say you chose wrong... PhDs in Physics take FOREVER.
But an MBA hardly gets you a PhD in economics...
In all reality, I agree with Musk, here. Being a physicist (even if just undergrad) gives you a much better leg up on spotting fundamental opportunities for improvement in technology than an MBA does. It really does teach you how to spot fundamental relationships and what really matters in a system, while giving you a broad toolset for general problem-solving.
I doubt you would have had as much of a fruitful time pursuing your MBA if you hadn't been trained extensively in Physics beforehand.