This is probably my favorite part of the game -- watching pro level players react perfectly to even the slightest tell.
So no, Starcraft is not perfect knowledge; much like poker you do a lot of reasoning and what amount to Expected Value (EV) calculations to figure out what your opponent is most likely to do. To gain information about what your opponent is doing in the game requires expending in-game resources, which is pretty much the exact opposite of perfect knowledge.
Starcraft does not have this, even with impossibly perfect scouting, so it is not a perfect knowledge game.
Actually, some of the recent WCS games are great examples; there's a reason why people will proxy or 6 pool. Sometimes it really does come down to luck, as in whether you scout at a specific time or not.
The difference is - in starcraft the metagame isn't stable - in a year buildorders that were very strong become almost useless (because of balance patches, better strategies discovered, and most often because the popularity of buildorders make them weaker - more people blind-counter them).