No, it's not irrelevant. Someday the site will change in a way such that it no longer happens to work in IE. But since they don't support IE, it won't be a bug.
Support is a commitment to people, not a mere technical hurdle.
Maybe. I wouldn't really bet on the site becoming completely unusable in IE, but I suppose it's a remote possibility. Let's be generous to your argument and call it a 25% chance. At any rate, you're right that it would be irresponsible to say they support IE if they aren't willing to fix it in such a case.
But because there's a 25% chance that the site could break sometime in the future, they go out of their way to break it right now? That's what they're talking about, and it's orthogonal to the idea of choosing what configurations to support.
At the company where I work, we don't support people trying to eat the newspapers we put out, but we don't go out of our way to make the newspapers poisonous either.
