If Arnold's story is right, there wouldn't really be a trap. Some firms are bound to be run by people who value profits more than pride in their alma mater. These firms will do what Arnold recommends: Hire talented people regardless of what diplomas they hold, and make money hand over fist. Faced with such competition, firms run by Harvard men for Harvard men would financially bleed to death.
...In contrast, if the signaling model is right, the Harvard men are, on average, really worth the extra money.
A lot of companies do hire on the basis of objective measurements rather than credentials, and they do make money hand over fist. I don't think that there is any evidence that Harvard MBA's help small start-up companies. Harvard MBA's matter for getting hired at consulting firms like McKinsey or at Wall Street mergers and acquisitions departments. It's hard for a new competitor to get traction in those fields. So I think that a credentialism trap in those cases could exist.
But admittedly the Harvard MBA is not the prime candidate for a credentialism trap. The most plausible example is education. In theory, there is a profit opportunity from starting a high-caliber university where you hire professors who are not Ph.D's. But try getting accredited, much less recognized as high caliber.