Yes money sitting on the bids is not necessarily the money sitting on the exchange, but:

1: bid sum is an excellent predictor of BTC prices. History shows that the two are very well correlated. If it's price that contributes to a rise in bid sum or the other way around is irrelevant.
2: what we are interested in is how much demand changes, not necessarily in absolute terms. So changes in bid sum over time should give us a good indicator of how how much money in total (even if not sitting on the books) sitting on the exchange changes too.






As you can see bidsum started to convincingly go up at the end of the bear market.
Also, we were talking $15-$20 millions at mtgox alone (now we are lucky if there is that amount on all exchanges combined). Bid sum continued to consistently increase in both bubbles.