All selling services and products around a very specific type of software, selling to a very specific market niche, leveraging both trademarks and non-GPL licensing to do so.
They're not selling the GPL software itself.
Take your Sourcefire example. They fund development by selling proprietary hardware and proprietary licensed IDS rulesets under trademarked names to an enterprise market niche.
It's also not true that all successful open-source companies sell to enterprises. Tenable sells Nessus to consultants. Sleepycat sold Berkeley DB to OEMs and developers. Automattic certainly doesn't sell Wordpress to enterprises.
Enterprise services.
The fact that they're also not actually selling GPL software (ie, an indirect business model), this all doesn't really disprove my point -- or the original authors.
http://news.ycombinator.com/item?id=211736
Red Hat is a cop-out example. Why not talk about SourceFire, Sleepycat, Splunk, Hyperic, Zimbra, and Astaro? These are all companies that sell software, GPL their code, and are (or were, before lucrative aquisition) extremely successful. SourceFire IPO'd off their GPL package; the company's name comes from the fact that they're open source.