Holy cow. I counted 4 - 5 folks beating on him . . . and he survived ?
I'm a bit confused how that's is or should be possible.
Sure it is possible. I got my Guardian up to rank 14.8 (almost 15

) and did this many times.
It is actually possible for any profession if you trait heavily in defensive and use defensive abilities. Guardians may be able to defend Conquest Nodes the best but it isn't significantly different than what any other profession could do - with the possible exception of Thieves.
If you want to try Conquest Node Defense and was wondering if you profession (other than a Guardian) can do it ----- this is my take on which profession would be better for Node Defense:
Guardian > Necromancer > Engineer > Elementalist > Warrior > Mesmer > Ranger > Thief
.... although the Thief would be the least suited for Node Defense - they would be at the top in terms of letting the other team take the node (and the thief hides in stealth) ----- then come right back to take it (with stealth and shadow step abilities).