Re-read my comment. If someone has an iPad 2 specific design which uses buttons which are 40 points wide on an iPad 2, those buttons will be less than 40 points wide when that design is inevitably served to an iPad Mini user.
If iPad Minis are served iPhone-specific designs, then you're right—it doesn't matter. And perhaps the solution is "you shouldn't be making iPad 2/iPhone-specific designs," but that's not what I'm getting at.
It is the exact same size as buttons on the iPhone if you are rendering the iPad 2 design on the iPhone as well, but in that case the button on the iPhone would be less than 40 points wide and also too small by Apple's guidelines. I am saying that it would be against Apple's guidelines on both the iPhone and the iPad 2.
That's why I mentioned an iPhone-specific design, as well. The iPhone's pixel density isn't a problem because it can be distinguished from the iPad 2. And, as you've shown, if the iPhone-specific design is displayed on the iPad Mini then there is no problem at all. The issue is that there is no way to distinguish between the iPad 2 and the iPad Mini.
If iPad Minis are served iPhone-specific designs, then you're right—it doesn't matter. And perhaps the solution is "you shouldn't be making iPad 2/iPhone-specific designs," but that's not what I'm getting at.