I think the unknown factor here is whether other benefits of keto over a standard American diet--possibly including reduced inflammation, BP, blood glucose, and body weight--balance out the effect of cholesterol. CAC measures actual damage already done, while cholesterol is just one of many factors.

The downside, of course, is that once the damage is done, it's done, so it's a risk. (And as you said, they won't see the damage in their 20s.)