But the # symbol is already used for tags (as your message shows). So it becomes difficult to differentiate between tags & topics. Especially with platforms that use either/both for different things. kbin is pretty unique because it offers both microblogging & link aggregation which adds an additional layer of complexity.
You could classify hashtags on other fediverse platforms as a sort of topic which one could subscribe to but how does that work with actual hashtags on kbin's microblogging. I think these kinds of decisions wont be made by a bunch of smart heads sitting together or chatting across fediverses platforms but rather some random developer that happens to be working on the one platform to get its UX down pat & break the mold.