That sounds like the Walmart silnylon to me. Looks like ripstop and is very slippery - check. Can't force water through it easily - check. Sounds like you got yourself some silnylon! I don't know if there are any other possibilities considering it's Walmart.

I stocked up on it a couple of years back and now that I'm retired I need to start sewing it into things (I did make a couple of daypacks, a poncho, and some stuffsacks).
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dk