I don't think it is Twitter, I think it was the requirement that people log in with their facebook account that destroyed Techcrunch comments. I personally saw the number of comments drop off greatly as soon as they put in the facebook login.
Another thing that's changed since 2006 is that I think publisher thinking about commenters has evolved to where they no longer see them as adding business value. Gawker made great attempts to court and build a commenting community on its sites from 2006-2009 only to decide that the commenters weren't really creating much value and were more trouble than they were worth. Now commenting is almost an afterthought on their sites and they don't really care to make it more readable.
i.e. Facebook comments might be a great fit for Techcrunch. They're dangerless and require little to no moderation.
By 'destroyed', do you mean reduced the number of comments, or reduced their quality? I'm not a TC regular, but I've never been much impressed by their comment quality, either before or after the adoption of FB comments.
As an aside, Eric was also a Y Combinator company founder a month or so after this article, though his YC company, WriteWith, was shuttered a short time after. Eric's a smart guy who works hard...so, not at all surprising that he'd go on to do interesting stuff.
