The conspiracy and accusations must always go deeper.

I'm also surprised it ended with the grandma realizing it was just math, because it could have just as easily ended with her thinking that they're obviously hiding what the real game is about, and how bad it must be be that they'd go to such great lengths to cover it up.

I have to assume having a good, strong relationship with her grandchild must also be a contributing factor. If D&D remained something only anonymous ne'er-do-wells do, it'd be easy to continue buying into the satanic panic. But someone you know and trust to be responsible telling you it's no big deal might make it a bit easier to accept.