There's nothing wrong with a Google employee participating in a Google-related discussion (edit: I mean in an HN thread of course), same as with any company. It's nice if they say so, which jefftk did.
Nothing wrong with that, sure, but making comments related to ongoing antitrust litigation against company employing you, while not being any official spokesperson and not having your comments vetted by legal and PR, can be really, really detrimental to one's career if Google decides that it doesn't like your comments. Google is very explicit in its internal trainings to never make any public comments like that unless you're officially empowered to do so. You can say or write anything you want on HN... on the last day at your job.
Yes, in the EU employees are more protected and it is much more limited what an employer are legally allowed to do. In fact they have to prove that your intention was malicious.
No, why would they? He's not whistleblowing. The commenter even admits that he has no context on the link in question and just suspects it's what's being implicated.
It seems possible that in the history of internet forums, some of them might occasionally receive subpoenas because forum members posted something essential to one side's case in some litigation. It also seems possible that some of those forums receiving subpoenas would rather not deal with the burden of responding.
Edit: I misread. Corrected below. Sorry!