No that's not always true. It's true that vorpal +22 can keep you alive when the monster level is above 200, but that depend of alot of factors. How much adrenaline you have, Damage Bonus, energy Leach level, your own level and what wave are you on. Plus you have to consider that some waves (like 15) are full of monsters equiped with shield that reflect Shock Rifle beam reducing the vorp chance. Also some other waves (like 13) are full of monsters that give few adrenaline once killed (think of the Sliths or the Raptors). It all depend on various factors. The moment in the wave where the monsters level became high also have an impact (a wave last 4 minutes, if the monster level increase after 3 minutes then you only have to survive 1 minutes, if the monster level increase after 1 minutes, then you have to resist during 3 minutes witch is much more harder). The vorpal +22 usually can barely maintain your adreanline meters on high levels monsters (I experienced it myself, you can barely keep your adreneline level by using globe+DMM on high level monsters in wave 13, and most of the time you are losing more adren then you are gaining it) and on wave 13 and 15 it's way more difficult to succeed then on wave 12 or 14 for example because the monsters there have less shield and give more Adren when killed.But the way the vorpal is setup now is that anyone can win. If an am with high enough ammo bonus, adrenaline, and adrenal surge manage to obtain a vorpal shock 22, they win regardless of the monster level and what map they're playing on.
To resume this situation a very few percentage of people can really succeed surviving monsters above 200 level with the vorpal +22. So in reality, even the highest level AM CAN DIE when that happend. It's all a matters of various factors end events that you can't control. And beleive me, luck got his part too.