Were you wearing it on your back or carrying it as a duffel? I saw folks with duffel bags of about the same size as the Aeronaut (though much uglier) asked to relinquish their bags to the cargo hold.
You have been far luckier than me. I've flown with the Aeronaut once, and in both directions I was told that I could not carry it on. In addition, the shuttle bus to the airport wouldn't let me carry it on either, which was really surprising! I've never had a problem carrying on my Brain Bag in any context, and the Aeronaut is really not substantially bigger, so the whole thing has been rather strange.