As I understand it the default is to give glucose, because hyperglycemia is unlikely to be fatal in the very short term but severe hypoglycemia can be quickly fatal. The patient will either regain consciousness or not.
However if you are not a trained first aider with an epipen of glucose (glucagon) AND of insulin (in which case you would also have a BG meter) then how are you going to treat them?
For an unconscious and unresponsive casualty you don't even know if they are diabetic. You also don't know if their coma is due to diabetes, even if you suspect that they are diabetic. They could have fainted from low blood pressure and hit their head.
So the base line is that if you can't reliably diagnose, then do nothing.
If the casualty is a T1 diabetic with epipens for both hypo and hyper events, plus a testing kit, then you might consider testing to see if BG is very low or very high. Even then unless you have been trained to inject both insulin and glucagon any messing around is likely to cause more harm than good.
Some information here.
https://www.diabetes.co.uk/emergencies.html.
If someone who is insulin dependent either T1 or T2 is concerned that they might become severely hyper or hypo then it seems logical to carry a tester, emergency insulin and emergency glucagon kits along with very clear instructions on how to use them. This seems to match your "no ambulance available" scenario.
The very last thing you want is to be unconscious with a hypo and have someone who has watched a TV drama a couple of years ago confidently give you a couple of shots of insulin because "that is what you do with diabetics". Not responding, must need another shot. Perhaps another, just for luck..............
I understand that some emergency glucagon kits also require pre-mixing. Not the best situation for someone who is not familiar with the kit. Already mixed and ready to go seems sensible.
If you find someone unconscious and wearing a pump, then I pass the question over to insulin pump users!!