To be fair, as Lxr points out below, big O notation is an upper bound on the growth rate of a function, and since 2^N grows faster (asymptotically) than (1.618...)^N, O(Fib(N)) <= O(1.618...^N) <= O(2^N) (with some abuse of notation), and the article is technically correct. You may be thinking of big theta notation, which requires both an upper bound and lower bound.

This example is helpful: https://en.wikipedia.org/wiki/Big_O_notation#Use_in_computer...