Except it wasn't mathematically impossible. At least 3 guilds cleared NiM DG in under 5 days.
Yogg-Saron "Alone in the Darkness" took about 40 days.
25 man Heroic LK took weeks as well.
I think DiLiH said they needed 150-160 attempts to clear NiM DG. While the 2nd boss in a 5 man Operation being that much more challenging than the other bosses was probably unintended (hence the 30 second enrage nerf) it was hardly impossible to beat pre-nerf.
If you look through the thread on it you will notice that it was someone else that labeled it mathmatically impossible. I have always referred to it as overtuned. Our video of it was originally labeled impossible but them changed to improbable. Also the original statement was referring not only to the encounter but the enrage timer which proved to be true as no guild (though 1 came very close) was able to defeat the encounter without seeing an enrage until they fixed the enrage timer. Most guilds invested 150-200 wipes to get it down before the change. Ours did not have the luxury or time to invest that many wipes but we did spend a good 4 hours on the fight before skipping. Impossible? no...improbable before the fix? yes.
Also I went back and checked. Only 2 guilds downed it in the first week.