What other things outside of Jap is there? Ainu doesn't count as Japonic, it's an isolate on it's own and theres very few similarities between that an Japanese. The only other thing is Ryukan and Okinawa which are based on Japanese anyway (iirc).
The only thing is really Japanese and it's dialects. The dialects are close enough to Japanese anyway. They use the same grammar for the most part and all come from the same origin (as far as we know) so I don't see why not. Some dialects in the west like Yamaguchi and Kyuushuu are even quite heavily influenced by Korean and I imagine the same would be true in parts of Korea. It's really a matter of saying, yes, Korean and Japanese clearly share the same roots.
Essentially, the family splits into Japanese and Ryukyuan. Ryukyuan is a group of similarish languages that are grouped together, really there are different languages on each of the islands, Okinawan being one of those islands.
As I've said a couple times before, no one really cares what we think. The current prevailing linguistic opinion is that Korean is a language isolate, even if that's total bullhockey. For ease of navigation, Korean should be a language isolate.