This is a common misunderstanding.
I'll start answering by telling how a normal utensil cooks food. At lower altitudes we know that the pressure is high. And, at higher pressure water boils competitively slower. Therefore, cooking at a higher pressure will obviously be simple. Now, when you go up higher and higher, you'll find that the pressure decreases, and therefore cooking food becomes hard at higher altitudes(since boiling point of water reduces).
Now, in the case of pressure cookers, they work by making a high pressure environment inside them. To be more clear, when you keep a pressure cooker on the stove putting the whistle on it, what happens is that the water inside turns to steam initially. But when more and more water gets converted to steam, the pressure inside the cooker increases a hence the boiling point of water increases. That means, it will take more heat for water to convert to steam and or food inside will get cooked with this heat.
Now again, as I've said before, a pressure cooker works by raising its internal pressure. And it's this internal pressure that helps in cooking the food inside. But infact, the pressure inside the pressure cooker does depend upon the outside pressure. To be more clear, the pressure inside a pressure cooker is the sum of the pressure mentioned on its whistle and the atmospheric pressure. The reason is that the atmosphere exerts pressure on all things on the surface of Earth. As a result, the pressure inside the cooker will vary with the altitude(i.e., it will decrease with increase in height). Obviously, the temperature at which the water boils inside a pressure cooker which is kept at Siachen glacier will be much lower than the temperature at which the water boils inside a pressure cooker at the beach. Therefore, the food will take time to get cooked at the Siachen glacier even though your using a pressure cooker.
You can find more details in this question asked at chemistry.stackexchange and in this faq on pressure cookers by hippressurecooker.
P.S. Sorry for that misleading mistake that I'd made in this answer. And thanks for those corrections😁