If you read the article, they mean they don’t support it only as a development tool.
Developers are supposed to be willing to download and install whatever tools they need to get the job done.
That's correct. And the websites their development tool creates and manages are IE compatible.
Moreover, their app is targeted at visual arts professionals, who are likely heavy Mac users anyway and will therefore have Safari as their default browser. And any Windows or Linux users can simply download Firefox or Chrome (or Windows Safari), and they'll be good to go.
Smart move on 4ormat's part, I'd say, especially given their measured user base:
Whenever you can forget to support IE, you should.