Humans are sexually dimorphic, which means that they have traits that skew in one of two directions, typically thought of as masculine or feminine, this is where the concept of the sex binary comes from. The poster is correct to tell you that no human actually has a binary sex though.
Some examples of sexually dimorphic traits might be:
Body hair
Genitals
Secondary sexual characteristics like breasts
Facial hair
Endocrinal systems, hormones etc
Chromosone karyotype
Now some of those dimorphic traits are pretty straightforward binaries, like chromosone karyotype, whilst others are spectrums, like facial hair, the presence of breasts, or how much testosterone you're running on an average day.
They all lean in one direction or another, which is binary, but how far they lean, and in what assortment, is not binary. It's much more complicated.
You can think of a person and their sex as a big spectrum with needles all over the place, some might be smack right on female or on male, and others leaning one way or another to varying degrees.
When we try to define someone's sex we're essentially eyeballing all those needles and trying to take an average, but it would be silly to suggest that someone is just 100% male or female, since there's so much information at play.
When those needles are calibrated in particularly uncommon ways, or ways that prevent the taking of that "easy average", we refer to that as an intersex condition.