I'm not sure, because when you create a new post you have to select the magazine and that's already enough to end in the right place so people that don't care about federation in theory don't have an incentive to use hashtags.
Am I interpreting correctly that if I were to, say, make some random comment about being sick as a dog in a microblog, that might get picked up by accounts interested in dogs? If I understand that, that seems like a disastrous oversight.
I guess the sentence you wrote would be fine if you don't use the hashtag dog but even if you did I'm still not sure it would get picked by magazine fetching that hashtag because you are already posting from "inside" a magazine.