This isn't necessarily true; it's move-specific rather than percent-specific. If you get hit with something like Falcon's stomp or D-Tilt (both moves that deal more than 7%) you will be put into hitstun, but if you get hit with something like Ness's Dash Attack (first hit is 5%, two and three are 4%) or Pika's D-Smash (2% for hits 1-6, 3% for last hit) you can get launched without stun if you are lying on the ground. It may have something to do with the game's "special effects" since people tend to get popped up with no lag from moves with fire or electricity (Raptor Boost, Falco's Shine, etc.) and they are known to have some jank effects on the game (Roy's counter trajectory, for example), but that's pure speculation.