rndc reports zone type "master" for built-in zones, but the stats channel reports "builtin", e.g.

% rndc zonestatus 100.100.in-addr.arpa
name: 100.100.in-addr.arpa
type: master
serial: 0
nodes: 0
last loaded: Thu, 13 Jul 2017 08:21:04 GMT
secure: no
dynamic: no
reconfigurable via modzone: no

I think rndc should also report "builtin", both for consistency with the stats channel, and so that a real master zone for the locally served zones can be distinguished from an automatically loaded one.