I simply quoted RFC 8375. It specifically called out that while RFC 7788 mentions ".home" (quoted below), it wasn't reserved, which ".home.arpa" aims to fix. But while you say "home.arpa is for HNCP", I also quoted RFC 8375 stating it's available for other uses as well.

> A network-wide zone is appended to all single labels or unqualified zones in order to qualify them. ".home" is the default; [...]