I'm not familiar with LG TV settings, but you want to make sure the TV is passing the sources audio codec through to the bar without doing its own encoding. On my Sony TVs, the setting is something like "Audio Passthrough"
The Bose App will tell you which codec the bar is receiving, so that's the way to verify if you're getting Atmos. You're correct that Truespace spacializes all content into a pseudo atmos listening experience. It does a pretty good job, but I've found that well mixed atmos content does sound better, more convincing on both my soundbar 600 and 900.