I read the Williams tech file, which said:
Pitlane & Refuelling Strategy
The pitlane length and profile (i.e. corners in the pitlane entry) contribute to the determination of the optimum fuel strategy. The pitlane loss at Monaco is approximately 17.8 seconds, the 14th most penalising pitlane in the Championship. To complete a normalised distance of 5km around the Monte Carlo circuit requires 2.58kg of fuel against an average of 2.42kg per 5km across all circuits this season, making the circuit the 2nd most demanding track of the year in terms of fuel consumption.
You had the fuel-consumption figure correct, only for the average consumption for the normalized 5km distance instead of per-lap. So, 2.58kg for 5km translates, as I said in my own sheet, to 1.72kg per 3.34km, which is a Monaco lap. This also results in the Jenson car being capable of running 22-23 laps laps, which, minus warmup, is pretty much what Ross Brawn said.