it is a bit curious, but for me the curiosity is what the difference is between an overpressure load (more than "rated for") and one that will actually blow the gun.
Personally, and from a practical standpoint, I don't think any of the .45 Colt caliber Rugers will blow up from a 28K load. Of course I have no proof of that its just my feeling, and I could be completely wrong.
Damage in a gun rated for 21K from shooting 28K loads? Possible, even probable. But a catastrophic failure, in a Ruger at that level? I'd say unlikely.
That being said, I'm not volunteering my New Vaquero or myself to test it.
I don't know what would be considered a proof load for the .45 Colt, but a proof load should be considerably higher pressure than the max. working pressure of the gun. And guns are expected to survive proof firing without damage.
Considering the business that Double Tap is in, I think the only way they would allow such a statement "safe for all Ruger revolvers" was if they believed it to be true. Or a clerical error left out some words, such as "large frame", and nobody caught it.