If an integer is drawn from an n bit space, it has n bits of entropy. Yes it really is that simple.
That's not what the (clumsily written) article is about. It's not about sampling the integers to choose an integer, it's about sampling the prime factors of an integer, as a measure of evenness of distribution of prime factors.
It's measuring the information in the prime factorization, the information in the number as a value.