Like the article says, it's not that the original author was both sexist and sloppy with language, although they were. The problem is that the sexism was actively reinforced in a public forum, first by a dismissive comment and then by an actual reversion. It would have taken no effort at all to make the correction; it was actually more work to reinforce the sexism. That means it was willful.
If one of my employees was being willfully sexist in public in a way that reflected on my company, I'd fire them too. Node.js is a really important project, closely associated with Joyent, and they've already seen some backlash from this. This kind of behavior could have real implications for the company's bottom line, and potentially even legal liabilities. Being inclusive is good business.
To reiterate: the original sexism, while dumb, is not a fireable offense. To actually go out of your way to be an asshole and practice discrimination while representing a company: totally a fireable offense. I'm not really sure why so many other commenters feel differently.