Perhaps more importantly why would one retweet a comment? Rather than a post?
The way content propagation works here is that someone using Website A follows a remote content source (either a user, or a group -- aka a "community" or a "magazine"), and the remote hosting website (let's call it Website B) sends all subsequent content from that source to Website A, where the requesting user can then view it. If someone from Website A was already following that content source, then they get to see all of the content that Website A had already received, and benefit from earlier users efforts. But if that person was the first from Website A to subscribe to that content source, then they only get future content.
It's very similar to a, well, a magazine subscription in that way. NatGeo isn't sending you their 150 years worth of back catalogue when you subscribe in 2023 (not that you should bother subscribing to NatGeo in 2023).
The 'boost' button republishes content, though. Posts, comments, whatever. Hitting 'boost' on a comment republishes it, and once republished the group actor (the little bot-like construct that functionally is the group) sees it as new content, and pushes it out to everyone following it. This means it will reach websites that started subscribing to the group after the comment was originally posted.
Boosting is how older content (where older basically means "from anytime before literally right now") spreads through the fediverse.