The cat's stomach is surprisingly similar to yours in this respect. Neither humans nor cats nor any other mammal produces cellulase, the enzyme required to digest wood. Therefore leaf-eating animals must digest food with the help of bacteria, usually in a part of the digestive tract which is used exclusively for fermentation. The reason for this is that cellulase is a complex enzyme and it is "cheaper" to let bacteria do the hard work.
Anywho, the bioavailability of raw marijuana is usually stated as low, but it really depends on how active the enzymes that decarboxylate THC-acid are. In cats, I'm not really sure about this. I ate almost a gram of raw weed once and barely got high, though i had quite a tolerance.
On the plus side, cannabis is as nontoxic as it comes, so your cat should be fine. Be careful in the future -- many things that are not toxic to humans (such as onions) are toxic to cats.