The mutual exclusivity of Discussions and Forum is due to a Fandom policy; not an technological limitation/restriction. During early stage adoption of Discussions, Fandom would allow wikis to enable Discussions without disabling Forum. However, this option was not official and was implemented purely at the discretion of staff upon request from the wiki's admins.
Due to the lack of parity between the features, Fandom eventually halted the conversion process. In the time between then and the release of UCP, multiple updates were released for Discussions. This made the old conversion script outdated. Fandom is currently working on an updated conversion script which will be applied to wikis that still have Forum. In the mean time, wikis that wish to enable Discussions may do so but Forums will not be converted until some later date.
So basically, there are two possible cases. Either the wiki was an early adopter that asked to skip Forum conversion or they recently (post UCP release) enabled Discussions.
Short version: There was a time when admins could ask for Discussions to be enabled and request that Forum be also left enabled before UCP was introduced. At some point, FANDOM staff just refused to leave Forum enabled and now you can't ever get Forum on a wiki that doesn't already have it.