"Requiring Microsoft to port Office to the Linux operating system would be a rather novel application of antitrust"
That isn't a likely or intended outcome. As the article states, the Chinese government tried and failed to introduce their own OS (Red Flag Linux), and due to that failure still have hundreds of millions of PCs running either pirated or legit copies of XP. With XP no longer supported, Redmond wanted a lot more for W7 licences, and the Chinese saw no reason to pay the higher price, or the ransom for extended XP support. The Chinese government already have the option of migrating to any one of a number of vanilla Linux distros and Open Office for next to nothing, but that's not what they want.
The purpose of this investigation is simply to put some serious pressure on Redmond to offer much better terms for W7, and if they aren't forthcoming to fine Microsoft the difference between what they charge and what Beijing want to pay, as well as to force additional costs on them. The probable outcome of this will be that the Chinese government get a much better deal on W7, the investigation will eventually close with minimal or no fines. The only remarkable thing is that Redmond have tried to hold out against the Chinese government for so long, instead of accepting the inevitable, and looking for what would be the most positive outcome after accepting that China are going to pay them peanuts.
An example of that would have been to offer China the price they want, but only for the flagging W8, which then gives W8 serious volume and (hopefully for MS) more traction in corporate markets. Instead, Redmond's stance has hardened Beijing's position on W8, meaning that the discussions centre either on the now five years old W7, or at best on having to give away W9. Having to massively discount your next operating system before it is launched would be a staggering strategic blunder for Microsoft, but they do have a history of epic strategic blundering.