True, but open source developers aren't known for the piles of money they're sitting on.
Besides, F/OSS has never been a huge threat to Microsoft. They're the ones that set the price of things like web browsers to $0, and they came mostly late to the party for serious server deployments (vs. IBM, SUN, Linux, etc.)
The plaintiff's assertion is not just that they are unable to compete with a free product, but that they are unable to compete with a product which, according to them, is only being given away for the purpose of harming competitors and establishing a monopoly.
No OSS developer can establish a monopoly, because the terms of the license permit redistribution. Thus while they may drive other vendors out of the market, they can not hope to charge monopoly prices, as anyone else could promptly undercut them at no cost with their own software.