DD sales is not going to buoy up CoD's destiny on the PC specifically...
We do lack exact numbers, yes.
However, they have implied greater than 2% for sure:

Source: http://twitter.com/#!/fourzerotwo/status/10185360222
At 2% of 25 million, that only gives us 500,000 copies, which couldn't apply to millions.
Now, I agree with your overall point that CoD sells significantly better on consoles than PC. I don't think sane people will argue that against you. It's just that taking retail sales in the UK's first week an extrapolating that to a full picture generally doesn't work very well.
By the same token I could note that the PS3 version of Modern Warfare 3 vastly outsold the 360 version in Japan, but trying to say that the Xbox 360 version is selling terribly worldwide due to that makes no sense, because it misses the fact that the vast majority of Xbox 360 version sales do not come from Japan.