They should make their own instance if they can. The devs cited Star Trek.website as the model for communities like fandoms. It is also cleaner as the instance can make several subs which have different rules and content.
The problem is that it requires money, either by the admins or through donations.
Just because one is open doesn't mean it should be used.
I also think that, compared to Reddit, there should be a more collaborative relationship between the mods and the admins because mods can choose their admins.