Yeah, you're totally correct. You can place a squad onto the Map, then move that squad into the building with a move waypoint (and your selected building position). What actually happens however, is that the Leader stands in the correct position, and the AI stand in whatever formation that the squad is currently in. This is NOT garrisoning a building. This is an AI squad leader standing in the correct spot, and the other AI just... well, they're not doing anything really, they're just standing in wedge formation or whatever.
What would be cool is for BIS to provide a 'Garrison' waypoint, much like they've provided a 'Join' or a 'Get in' waypoint (you know, the same ones we've had for like what.. 5 games, or 11 years now?) to Garrison buildings.