It's an instead, so it stops anything that would make you loose the game
For Example, if your library has zero cards and you are forced to draw a card, this effect would trigger, you would go back to 20, and the game would continue. You would obviously loose the next time you would draw a card.
What I want to know is if this ability uses the stack in any way. If it does, and you receive 10 poison counters (which is a state-based check for loss of game as soon as any player receives priority, so if there was a spell that said "gain 10 poison counters, then draw 10 cards, destroy 5 lands, then shuffle your hand into your deck and draw 1 card", you would do all of the actions before the game checks if you are dead for poison), you could in theory have assembled some contraption that lets you create copies of this creature at will, which each time the state based check happens, would allow it to loop because of the exile trigger. Would a judge even allow this to happen?