In this case, willing to proceed with what he was planning to do, that is, moving from Windows to Linux against Microsoft's wishes.
edit: And you're right that this implies "balking at Microsoft's proposal". Personally, I did read the sentence as intended ("they wanted him to balk (= stop his plans), but he did not"), but I can see how the phrasing can be misleading...
In this case, willing to proceed with what he was planning to do, that is, moving from Windows to Linux against Microsoft's wishes.
edit: And you're right that this implies "balking at Microsoft's proposal". Personally, I did read the sentence as intended ("they wanted him to balk (= stop his plans), but he did not"), but I can see how the phrasing can be misleading...