Yes, that is correct - it is NOT a PPLNS pool. That's why I said "like a PPLNS". It was a bit easier to explain it that way.

For those looking for the more-detailed definition - look at the top of this page: https://en.bitcoin.it/wiki/Comparison_of_mining_pools. And Slush's scoring system is outlined the home page and detailed in this message: https://bitcointalk.org/index.php?topic=1976.msg50002#msg50002.