Indeed, Sound Check does not compress music, it just adjusts the gain (volume) as needed. It works pretty well, but (like the equivalent on Spotify) it's not a PERFECT solution and you'll still have some tracks sound quieter because they're otherwise mastered so loud that the gain correction turns them way down. This should actually encourage mastering engineers to stop brickwalling music (the classic Loudness Wars stuff), leaving plenty of headroom and dynamic range, and sounding good on streaming services which have volume normalization.