It’s not, it’s a subject. It’s what the song is about, not what the song sounds like. A genre categorizes music based off third general sound. These genres can have certain subjects more or less relevant to that sound sure, but it’s not their entire identity. There has been Christian music in all genres
Rap: Toby Mac, NF, Forrest Frank
Rock: Newsboys (Old), Sanctus Real (Old)
Country: Anne Wilson, Zach Williams, Crowder
Electric: For King and County, Tauren Wells
Ok everybody else is sorta the same but you get my point
Music categorization isn’t a perfect or comprehensive thing. We categorize based on what people find helpful. And there are a lot of people who listen based on subject, at least in this case.
Another example is “Christmas” or “Holiday” as a genre. It’s a genre based on topic, not instrumentation. But it’s a useful delineation.