"Public keys are sized so that, absent a breakthrough in solving the discrete logarithm problem, brute force is impractical for any conceivable amount of computing power. With 2^128 possible combinations, and if we assume a modern computer can compute, say, a billion per second (which is a massive overestimate), it would take a million such computers ~5e15 years to brute force your key. If we assume computing power improves by another million fold, it'd take this massive cluster 'only' about 5 billion years."
However, according to this question, a private key is 256 bits long. In this case, why is the quote referring to 2^128 instead of 2^256?