Does it mean you know your site is broken in the browser? Does it mean you block that browser? Does it mean you intentionally broke your site in that browser?
None of the above. It means you don't want to waste your QA time testing and fixing minor quirks in the browser and you don't want your users having a degraded experience on that browser. To protect your users, you may or may not provide a warning or block the browser entirely.
Products are in constant development, and every CSS change or javascript feature has the potential to break a browser and require fixes. Just because it works now, doesn't mean it always will.
It appears they successfully attempted to support IE, but decided it was not worth continuing, perhaps because of some breakage OP did not discover. Should they have stripped out any IE supporting code? That seems unnecessary.