Your hormones shift later at night, so your nervous system is in a relaxed state. Your improved breathing and heart rate let you push harder.
I don't think it has to do with being a "night owl" as much as noticing enough to take advantage of something that happens to everyone. A lot of people aren't curious enough to change things up and that's probably who this article is aimed at.
I don't think that's necessarily why, as I know people who feel they can push harder in the mornings compared to later in the evening. Those people I know have always been early risers and hit the pillow early.
I don't think it has to do with being a "night owl" as much as noticing enough to take advantage of something that happens to everyone. A lot of people aren't curious enough to change things up and that's probably who this article is aimed at.