Calculation of slope requires measuring vertical and horizontal distances and it's important that they be in the same units. If your DEM is not in a projection that uses the same horizontal units as vertical (e.g. if it is in geographical coordinates, which in this case I believe it is) then you need to use a non-one valued conversion factor. There's an excellent website on the ArcGIS Help documentation that reviews the various common z conversion factors for vertical/horizontal unit mis-matches:
Another ESRI blog site suggests the following for geographic coordinates when elevation is in metres:
zConvFactor = 1.0 / (113200 * Math.cos(Math.toRadians(midLat)))
To tell you the truth, I'm not entirely convinced about the accuracy of that equation...particularly the 113200, but it certainly works with similar tools like Hillshade.
And one last thing, topographic slopes are not going to be higher than your 89 degrees; in fact, there are very few places on the Earth's surface with a slope anywhere near this steep. Even shear cliff faces that appear to be extremely steep when you're walking along them likely have much shallower gradients than that.