Hello ... I can't answer your question about how the device works out the slope / distance but unless you have different rules in the US you aren't allowed to use this facility in competitions? - Why bother with such a device? - I have a Leupold rangefinder & it has been working perfectly for the 3 or 4 years I have owned it
Battery lasts about 6 months & as I play at least 3 times a week, that is pretty impressive
Good luck ... Paul
Most rangefinders that have slope capablity have "accelerometer based inclinometers" that measure slope angle and then compute the estimated distance , basic geometry, and then let the golfer guess which club to use. You probably have the inclinometer capability on your phone and in the past I have downloaded an App to test slopes on some of the greens.
My Golf Spy did some testing of range finders in June https://mygolfspy.com/best-golf-rangefinder-2018/ and you can tell from the comments that not everone agrees about the final positions of the items tested. What is "Best"? I still use an old Bushnell V2 and it does what I need it to do.