the author invents whatever he likes ?

To a certain extent, yes. The stories (it's one of a series) are about WWII (The Great Patriotic War) proceeding merrily along its way when the aliens land thinking they're going to walk all over us in a couple of months based on observations they made 500 years ago. The author was perfectly free to make the aliens think and act any way he wanted, but he should be limited to known reality with respect to the humans.