Garlic will burn after about a minute if you cook them alone, but being mixed with onion distributes the heat, plus onions release liquid as they cook which also prevents burning. Depending on how much onion and how hot the pan is, it’s not always going to burn the garlic. It’s good advice and it’s something to be aware of.

In this case the two are separated so the garlic will finish way faster than the onion unless they were about to mix it.