That wiki was likely created before both forum and/or discussions were not default yet, and because of that are not (yet) subject to the forum to discussions migration. There are still a few of those around. That migration is different from the full platform migration, has other criteria, and I believe "has to have discussions in order to be migrated" is not along those requirements either way.
Forum to Discussion migration has to happen before legacy to UCP migration because UCP doesn't have Forum. While it is true that legacy wikis may not have had Forum, I don't see why Discussions wouldn't get enabled as part of the UCP migration process.