This sentence brings up one of my pet peeves. The first time I did this lesson, I dutifully wrote, "One is less than two." But whenever one is comparing numbers, such as ten cars compared to twelve cars, the proper term is "fewer", not "less". I tried answering with "fewer" and was marked wrong. I realize that there is no context (big surprise), and therefor the possibility that the sentence is referring to quantity, such as, "one litre of water is less than two litres of water", but it is equally likely that it is talking about horses, or cats. So it seems to me that both "less" and "fewer" should be accepted. I apologize for the length of this rant, but as I said, it is one of my pet peeves.
You are right that "fewer" is correct when comparing countable objects, but there is no object in this sentence.
In mathematical expressions, "less" is the proper usage. "One is less than two" is correct because the implied comparison is the magnitudes of the two numbers. "[The magnitude of] one is less than [the magnitude of] two".