You nailed it. For someone who needs to determine the endianness of a machine on the fly, the question is trivial to solve; for everyone else, it’s trivial in the sense of “unimportant.”
Their pre-screening quiz uses a bunch of questions on various technologies to ensure that some subset of the questions tests you on technologies you're familiar with. Which is good, since not everyone is a web engineer.
I don't think there would be too much correlation between CS degree and ability to answer that question. There are lots of CS degree programs that don't deliver such low level instruction, and there are plenty of people without CS degrees who can answer that question because they were curious about how computers work or have been exposed to low level programming.
