Disclaimer; this is only based on my own experience and thinking and not any science.

In general hanging on to slopers involved maneuvering your body and hence weight IN and UNDERNEATH the sloper as to gain the best possible angle of pull. This is most often accomplished by use of technique and specifically footwork (heel- and toe-hooks, pulling with your toe etc). In these cases your are often hanging on the slopers by way of friction, more then you are actually pulling on them.

Actually pulling on slopers is often a whole different game. Consider hanging from the 45 degree sloper on the Beastmaker 2000 with and without a well placed toe-hook. With the toe-hook you would be able to more easily hang on the sloper. Without the toe-hook you would have to be able to pull on the sloper to keep hanging. This is where "sloper strength" for me comes in. Does this make sense?

Counter-intuitively I find that effectively pulling on a sloper involves more pushing then actually pulling. To emphasize this further take a look at this video of a guy actually hanging from the B2K 45 degree sloper: https://www.youtube.com/watch?v=94FSBbVf7yA

It would seem that he ever so slightly maneuvers his body IN and UNDERNEATH the hangboard. This can only be accomplished by pushing in addition to pulling. To me this closely resembles how you perform a front lever. When doing a front lever you push down on the bar or rings to elevate your body underneath.

It stands to reason that training front levers and possibly other gymnastic strength exercises that develop this displacement of bodyweight would be beneficial in developing "sloper strength".

That said, the best sloper strength will always be a result of good technique as you rarely do moves on slopers without any option of using your feet as a way of effectively maneuvering your body into the best possible pull angle.