Obviously, Salyut 7 must have been in a considerably lower orbit. Slylab was in the upper levels of the Thermosphere where the temperatures are around the 1000C and above range. Salyut 7 must have been lower orbit closer to the Mesosphere where the temperature is much colder -100C.


That had nothing to do with it. Skylab lost solar shielding during liftoff, causing it to absorb more heat from the sun than intended. This caused it to get very hot inside. Salyut 7 lost electrical power causing the internal equipment to shut down. This upset the thermal equilibrium (heat loss became greater than internal heat production) and caused it to get very cold inside. This was similar to but more extreme than what happened with Apollo 13.