Another issue for niche forums is cost. They may have the financial resources for managing their little group, but ActivityPub requires significant resources and adds significant overhead, that the little group of people might not be interested in affording.
And then there's the issue of niche groups showing ads on their sites to fund their servers. ActivityPub risks undermining that source of revenue.
I'm not saying one way or the other since it's definitely a case by case decision to make.
Although I will say this is one place the overhead and cost of ActivityPub comes up, so I'm critical of that.