I don't think this was the original source, but this paper matches your (& my) recollection: https://gwern.net/doc/cs/cryptography/2006-arkin.pdf

google surfaces a couple of HN posts, but the source (cigital.com) seems to be a dead domain at this point:

https://news.ycombinator.com/item?id=288138

https://news.ycombinator.com/item?id=9914607

It's a bad shuffle implementation + using time of day as seed (reducing search space). Using the player's 2 cards and the 3 flop cards, it finds the RNG seed in real time, and then future hands (on the same server) are solved in "under one second!"