Because the Alky isn't changing the air mass that passes through the MAF. It keeps it at a manageable temperature after it goes through the supercharger. Which it will still expand reasonably, but not as much as if you're trying to cram so much air into the engine that you're boost stacking. If anything, it's acting like unmetered fuel. Which is part of the reasons I've read about why it's so difficult sometimes to tune a car with alky. You can still run smaller pulleys after running methanol, but you've still got to keep your fuel trims in check regardless what size pulley you are running.
