Well, losing by any competitive group, gives you the opportunity to just go back at it and play better, improve group-play, positioning, blah blah blah. Nothing's undefeatable. Losing to a zerg where the ratio is ... ahum.... well, there's nothing that can be done there (there used to be a zerg farming era but that was when most of the people had shite resists stats armour etc)
Utter rubbish. If you are running in some PuG there are many occasions when you won't stand a chance with even numbers against a good set group. However its perfectly possible for that same group to fight a zerg. Obviously you can't farm the entire zerg but its possible to hit and run.