I think a) because adding a 20 minute step to a recipe makes it unappealing so calling it 10 makes the recipe more popular, but more importantly b) because what they mean is "make them transparent and possibly slightly brown" which you can sort of do in 10. No one cared that this isn't really caramelizing. Very few recipes really need caramelized onions. They're a sweet mess. Basically I think recipe writers just like to throw around terms too loosely to make their recipes sound fancy when what the 99% of recipes including fried onions needs is "lightly fried, transparent looking onions".