I get this too, having up and downvotes is like "speaking the language" of Reddit and would enhance adoption through familiar UX. That's a big positive for something that aims to serve as a Reddit replacement.
At the same time, it's a headache to translate into ActivityPub, because as OP said, upvoting and boosting are very different concepts. They both aim to increase visibility, but an upvote differs from a boost in that 1) an upvote produces a gradual increase in visibility; 2) you can opt to ignore vote counts entirely, for example, by sorting by new posts; and 3) the source of an upvote is (generally) not public, whereas you're always told which user has boosted a post.
Sam_uk