Earlier quoted context omitted.
Except that many IE8 users don't even know what a browser is. That isn't an euphemism, they literally don't know the word "browser". There message as displayed conveys the message better than "We don't support your browser" to those people, and there are more of those people than there are Opera users.
That line of reasoning almost works, except for the fact their error message already says browser.
Obviously they could have written a paragraph of text explaining that, but just saying "something you are using is so old that it is lame (in the duck sense, not in the teenage slang sense), here's how to fix it" gets the job done in one sentence.
This isn't a business website, this is a social network that gets most of their traffic from bands now. I can't fathom how the smallest amount of snark about IE8 being old on a product like this is raising so much anger on the HN community.