Actually - that's not quite true.
As the OPK is designed for a manufacturers product (in this case, Microsoft, as any other OPK build for any other manufacturer), Microsoft does test and certify for the platform specific driver. Depending on the relationship with that component manufacturer, they can also define abilities within that driver set that are custom to their platform (in this case Surface series) - and test to that level of performance. In short - the Surface Series is a Microsoft hardware platform as are HP laptops, Dell, laptops, etc. are to HP, Dell, etc.
For example - simply look at the Microsoft certified driver for the Intel 4000 or 4400. Certainly you yourself are intimately aware of the differences between the Microsoft Certified Version and the non-OEM Intel version.
As the OPK is designed for a manufacturers product (in this case, Microsoft, as any other OPK build for any other manufacturer), Microsoft does test and certify for the platform specific driver. Depending on the relationship with that component manufacturer, they can also define abilities within that driver set that are custom to their platform (in this case Surface series) - and test to that level of performance. In short - the Surface Series is a Microsoft hardware platform as are HP laptops, Dell, laptops, etc. are to HP, Dell, etc.
For example - simply look at the Microsoft certified driver for the Intel 4000 or 4400. Certainly you yourself are intimately aware of the differences between the Microsoft Certified Version and the non-OEM Intel version.