'Member of Congress' can be narrowed down pretty quickly simply to a comparatively short list by considering which Senators or Representatives are slightly likely to be interested in reading about mostly Californian tech companies on a website called Hacker News.

That anonymity disappears much more quickly if said Member of Congress has also posted in the past about where they're from and which political issues they're passionate about...

It's a small enough category and people will be interested enough that there's a good chance that a dogged reported could figure it out in a week given a long posting history. I'm pretty sure that if I admitted to being, e.g., the Zodiac Killer or something and people believed it then the FBI would probably be able to figure out my identity even without the link to my Github in my profile. And more importantly you can never be sure that you haven't let something slip that might reveal your identity in a way you didn't anticipate.