Some image data like the Moon SLDEMs from https://pgda.gsfc.nasa.gov/products/67 have scale factors applied for image compression. The slope images are supplied as Int format and the label states to multiply by scale of 0.0015 and offest of 45 to convert to slope in degrees.
Often this metadata is machine readable and applied automatically but in this case QGIS doesn't see it.
To manually attach the scale/offset with gdal we can make a VRT (a simple "virtual" image that doesn't copy the original but points to it and adds this metadata):
gdal_translate -a_scale 0.0015 -a_offset 45 SLDEM2015_64_SL_90S_90N_000_360.JP2 tmp.vrt