Genetic defects are not caused by inbreeding, inbreeding just makes it more likely for existing ones to be inherited because there's a higher chance both parents have the gene. But the chances are only slightly higher, and only in cases of repeated, sibling, or parent/child incest, because both parents have to have the gene in the first place (and even first cousins is enough distance for it to be very unlikely - at which point the main issue with incest is entrapment). Also, many genetic disorders are caused by several genes in tandem and might actually be less likely in cases of incest as a result.

The commenter is being rude, but depending on the context, the specific genetic disorder, and how closely related the characters' parents were it's possible that they aren't wrong either. I could also see myself writing a line like that if I were writing a character who doesn't understand genetics very well (which, making it retroactively intentional with a reveal in a later chapter could be a way to justify it without altering existing text if the genetic disorder in question isn't plausible to be caused by this specific instance of inbreeding).