I believe that fire brigades have a device able to display thermal gradients of the airframe which might help in evaluating the source/location of a fire.
Nevertheless "evacuate or not" decision, is one of those grey zones for which, if nothing happend, people will say it was a good decision, but if it blowed up, then we will question your judgement.
Like mentionned, not opening the cargo door before pax/crew have disembarked, it is a normal procedure, as it depend on the cargo you carry,e.g.Radioactive, and fire brigade must be present.
I believe that detectors are not 100% unbreakable and that they are not always 100% reliable depending the load carried (cargo plane).
If you have a fire and that you evacuate, then legally nothing should happen. Despite having maybe your company questionning your decision.
If you land due to fire and that you dont evacuate based on your assumptions, you might be legally accountable in case of issues.
As written in the Boeing QRH " In all situations,the captain must assess the situations and use good judgement to determine the safest course of action".
I believe that if you base your decision on the elements you know for sure without forgetting that,e.g. if "smoke or fire cannot be positively confirmed to be completely extinguished,the earliest possible descent,landing and evacuation must be done", you might be not too wrong.
For the flight involved, i am sure that they have taken a decision based on the elements they had at hand. Good or not, i m not here to judge. I was not there.
Last, installation of thermal cameras might be of great help in assessing the best course of action when smoke,fumes or fire might be involved.