Steam has no motive to fight the subpoena, since they're not named as a defendant. They will probably alert the users that a subpoena was received and Steam is compelled to reveal the information by a specific date. (If their personal info isn't on file, IP addresses can still be gathered and used to subpoena their ISP.) The users would have to file court documents to fight the subpoena before that date, which usually requires revealing their personal identity (through court documents). So, it's a catch-22.

IANAL either, but I've read several cases where subpoenas were granted in relation to torrents, going on nothing but a connection to a specific IP address, which is far more abstract than an "allegedly" damaging forum post made using a pseudonym.

Now, the biggest problem with lawsuit harassment is that most cases never make it to court, so they're never actually "tested." While the case sounds absurd and would likely not go in Digital Homicide's favor, all they need to do is offer a settlement for less than it'd cost to fight the case (which could still be a few thousand dollars). For most people, the time and stress of going to court is worth settling over. Lawyers know this and fully abuse it.