Did you try Sly's teleport and/or counter move. I'm not convinced it'll work with those moves of his.
There is NO reaction time when on this ledge. It happened to me in the middle of the match and I could counter out after two combos (that was the first thing I tried when I first got caught). When on the ledge, you cannot get any chance to press anything between hits. I've tried literally every escape tactic in my arsenal when stuck, and on the corner none of them work.
I guess you don't know about the whole roll dodge from being down. There's no infinites here.
I know about it, but you don't get knocked down from this combo in this spot. If it were that simple, I'd've done it. This specific attack just stops you from being able to counter or get out of it in any way, because you can't block/dodge as Sly, the roll doesn't work because you're not downed, and the attack is so fast that it's not possible to get a button press in when on the ledge.
In a regular scenario, this combo is easy to get out of, because for some reason when not cornered you have a very small fraction of a second where you can use Sly's counter.