I think that Tiny are refering their own licensing arrangement with Microsoft. They will install from their master disk, which will be 'locked' to their special code. If you use your own Microsoft disk, then it would be your supplied activation code that would then be required.To use their code, so as to 'unlock' the system, should you have need to, then it would mean that extra purchase for the reload disks.
This as been a problem, that as been raised before. Extra money for Tiny.