Netflix hasn't done this (and likely won't) because would make it difficult/impossible to control the quality of their service. P2P connections are a bit of a crap-shoot based on things like router configuration, device type and resources. For example, P2P wouldn't work as well on an iPad where you're only caching a few hundred megs of video (and not the whole video) at any given time.
I don't think the parent meant to imply a P2P implementation of Netflix streaming video, just that Netflix is perfectly capable of making that ratio balance out by uploading crap from the clients they have (partial) control over - which would then be traffic coming from Comcast's network.
