Holding Jab has gaps in it that allow Ledge Climb to beat it sometimes; that's not a reliable defense against it and letting go of Jab after you see that the ledge option they chose isn't Ledge Jump leaves you vulnerable if they Ledge Climb.
D-Tilt's incredibly fast animation seems like it would work to cover both Ledge Jumps and Ledge Climbs simultaneously by reacting to the opponent moving from the ledge at all. Most Ledge Jumps are vulnerable at around Frame 12, while most Ledge Climbs are vulnerable at around Frame 32. D-Tilt hits on Frame 3 and is finished on Frame 21, so a well-timed D-Tilt will hit a Ledge Jump user into a combo position and will end in enough time for you to use another move (likely a Jab or tilt) to catch a Ledge Climb. It also has the advantage of having great range, so opponents using a Ledge Drop -> double-jump aerial will trade with you at worst and lose to D-Tilt at best. With good timing, this seems like it's one of the best ledge coverage options in the game, period, and with Forward-B having increased knockback in the 1.06 patch, this could be a strong game-ending thwomp on the last stock.
Jab being Frame 1 means it should be a really good tool for catching the end of a Ledge Climb on reaction; that can lead to a lot. Full Jab combo for damage and Jab1 -> Jab2 -> Up-B as a hit confirm into a KO move both work from the advantaged side, while rolling behind the opponent and doing a Rapid Jab slides you off the level after six rapid Jab hits, allowing stuff like Rapid Jab -> Forward-B and Rapid Jab -> KO Punch.
People being afraid of our superior poking tools often leads to a lot of shielding post-Ledge Climb, so a well-timed grab does good, with two pummels -> throw dealing solid damage. D-Throw/F-Throw is a decent DI mix-up in that, too; an opponent can escape a D-Throw -> Up-B by holding away, but F-Throw sends at a sharper angle in that case, sending them further away, which can be great for resetting the situation and for edgeguarding (which is heavily matchup-dependent, obviously).