I spoke too soon, I think that IA is having some intermittent problems, but I was finally able to get to Biden's "National Strategy for Countering Domestic Terrororism" here:

https://web.archive.org/web/20210617103741/https://www.white...

At least in the broad-strokes PDF I see no mention of "radical Christian" anything. If there's some citation, I'd be curious to see it if it can still be found and if it mentions specific groups.