It’s pure conjecture until the commenter posts some sources. Now I feel patronizing pointing it out, but you don’t have to stay up late to have sex. I think this is modern cultural norms being propped with pseudo science.
That doesn't mean that having a 'late night' trait wasn't advantageous to mating. For example, when humans started pair bonding, infidelity may have been more common among those who could sneak out under the cover a darkness while their partner was asleep, spreading that trait further than those who remained monogamous. It is easy to think of many possible reasons why a late night trait could have been advantageous in spreading that trait further than those who naturally followed a more daytime rhythm during teenage years.
Or it could just be random chance. Either way, it's fun to discuss. We don't have to dismiss all fun discussion because it may be conjecture. It is only the comments section of HN, not a respected scientific publication. Time and place.
It is fun speculation, but still, OP's self-assured "this is so trivial" tone was not called for.