Also, what impressed me about the Disqus incident, was how fast it came back with that list.
In the US, at least, true anonymity takes a lot of work. For example, if you own a house, people can use tax records to find out who you are, unless you do what rich people do, and use shell companies. I also own a couple of [small] companies. I maintain a UPS box, because they get a lot of junk mail (and some business junk mail comes to my home address, anyway).
That's just one of hundreds of ways we can be found. Many predate teh Internets Tubes. My mailbox gets stuffed with junk mail. Some of it is quite specific. They use these mechanisms, and have been, for decades. I have known folks in the collections industry. They can find people surprisingly easily. There was one guy who used to be a skip tracer, and he wrote a book called How to Disappear. It's a fairly sobering read (and probably quaintly anachronistic, these days).
The Unabomber actually did it correctly. He only got nailed, once he posted something publicly.