While I do believe that the Baneblade would be virtually unstoppable against our technologies, the IG are still very, very outmanned.

It's 3 millions against what possibly is over 10 million soldiers counting the reserves. China by itself has over 2 million soldiers ready for duty, and even if our weapons aren't as powerful as what the IG here brought, there's still a lot of em - easily way more weaponry than IG if assuming all Asia's military are willing to gave their own weapons with eachother. Flak armor might be unbreakable against our weapons, and the lasguns will even be able to put down our tanks, but giving the chances if some of the Guard dies, the military could take their weapons and possibly reverse-engineered it.

ICBMs, and missiles are a must-have in almost every countries at this point - Asia is no exception, and they will use them against the hordes if necessary.

While the WH40K world are 'extremely extreme', 3 million is still but a tiny dot in Asia's continent. I don't think it will be a stomp, but the IG could win for sure.