If they reason that a policy letter supersedes written regulation that's fine. We also have a policy letter from CSAF saying stop doing dumb shit. Who adjudicates which letter wins? Commanders. Not their front office or worker bees.
This should, clearly, fall into the realm of dumb shit since you wouldn't be in compliance with the policy even if you execute their proposed COA (for arguments sake, assume you get <90). I recommend elevating this past the front office to an actual commander, and presenting it as simply as possible: I can't take the test early and still meet the requirement, so I'm planning to take the test a KRND. If they say execute, do so. Understand your desire to avoid appearing whiny, but you can attack this very nonsensical policy interpretation in a professional manner (which you're already doing); if folks are offended buy that, it's not your fault.