Uh... it doesn't cape through the invincibility frames... and ASL = Auto-Snap Ledge.
http://www.youtube.com/watch?v=v10373amNZY Watch 0:26 and notice how Marth's body is above the ledge. In order for Cape Teleport to work the cape has to hit, and the hitbox isn't wide.
It's also not Jab -> D-Smash. It's Jab-Jab -> D-Smash.
The frame data showed here shows that if both jabs connect, Mario has a +7 frame advantage to perform any attack before the opponent can do anything. Legit setups are D-Smash (Frame 5) and F-tilt (Frame 7, have to buffer correctly).
Why isn't this used alot? Because the jab has to be sweetspotted, which is the front of the fist. This means Mario has to space his jabs correctly for this work, and Jabs are typically used as a GTFO move, and F-tilt is better for spacing since it can poke out of grab range and can be used consecutively before the opponent is able to do anything outside of 2-Frame jabs.