The policies around blanket ambiguity for rejections is to avoid any kind of messaging that could lead to potential legal retaliation. Frustrating, absolutely, but most employers just aren't willing to flirt with the risk.
With any job, it would be one thing if it were at the applicant stage, and I hadn't talked to a person at any point. But with this one, there was an offer in front of me and there was no one at all who had both the capacity and the willingness to explain what had just happened.
If I started the job and they pulled this on day three, they would have to give a reason to an unemployment office.
I don't care how little inclination businesses have to tell the truth. Make them commit to the lie, in writing, somewhere that it actually costs something.