It’s explained better in the novel, but Chase was so traumatised by being an alpha/ruts/sex- the dog allegation- that he was averse to learning/hearing anything about sex. Chase says it himself, you the know the stuff about how he hates being an alpha and having no control over his body. So he doesn’t know that only omegas get slick. And because he believes Josh is a beta, he thinks there’s no logical way the scar is from a c-section (also Josh is covered in scars, but the art is only showing this one). Once he finds out Josh is an omega, he instantly clocks that it must be a c section scar and that Josh himself had given birth

Ong like how can you be so stupid???? C-section scar? Slick?? Like he is clearly an omega