It's a high efficiency system. That's what they call for. Instead of venting into the chimney they go directly outside, one is for intake of fresh air and the other is for exhaust.
I would have figured the exhaust would be hot enough to melt the plastic. My understanding was that you have to have metal for the first X feet because that's what would be where it's hottest.
But what do I know? If they say you can use plastic from the beginning then have at it. It's cheaper and easier.