Many Indian recipes give the requirement of simmering the gravy, covered until reduced. I'm wondering if this advice is flawed as one would presume the pan would need to be uncovered in order to reduce the liquid.
It depends on how long the recipe calls for, and how much reduction is required. Most Indian recipes I make don't require much reduction, they are relatively thick to begin with. Lids don't seal perfectly, you will lose moisture even with the lid on. If you have a long, slow cook it may reduce enough for some recipes.
Some recipes do call for a slow reduction of a sauce, especially if the sauce contains something that requires slow, gentle cooking (usually stewing meat of one kind or another). as Chris H has commented, that would be with the lid cocked, or not perfectly sealed. It saves the trouble / utensils taken in separating whatever has cooked in the sauce, reducing the sauce rapidly, and recombining for service.