If he wanted to argue that, he probably should have argued that in court. Especially considering that he always had the chance to do so. But that would be admitting guilt, isn’t it? He continued to fight the legal system after he was shown to be squarely in the wrong. Indeed, this is the case where legal system is actually working as intended - Judge shouldn’t actively protect either side, they judge, not represent.
And yes, ‘Being bankrupt’ is a valid argument. But only if it is true. Just like any other arguments, if evidence to the contrary is found, that is just another crime to be punished for lying in the court. Now you can see how he ended up with that much fine.
I would also wager that Valve was worried about Microsoft attempting to use “creative” methods to compete with Steam and chipping away at them, like hidden API. Its not like Valve knew that Microsoft’s attempt would continue to flop so hard for decades that they couldn’t even try that.