I had read suggestions that it was a refreshing of the post into the feed, so an old or new post that got boosting would get new life for visibility.

This is true for the microblogging services, but boosting is pretty much pointless for kbin/lemmy users (right now). On the microblogging systems, you have two signals you can send about a post, like and boost. A like doesn't affect the post and just lets the author know you liked it. A boost literally boosts the posts and presents it to your followers, even if they don't follow the OP.

A like is mapped to an upvote on lemmy/kbin. But the visibility boosting is probably why kbin originally mapped upvotes to boosts, since they're both about enhancing visibility. The problem is lemmy already used likes and microblogging users dont like being overwhelmed with boosts and users outside a forum are more likely to like a post so like <-> upvote allows non-forum users to affect voting.