Each streams user has a delivery report of each message or comment sent, where the names of the people and servers who receive our messages are written, and these are sent exactly to the people who have a relationship with us (our contacts) or to the people described in the circle (case of non-public messages), if these messages arrive at other servers it is not our fault, but rather it is another software/server that sends to other servers. If the messages of people we follow (but who don't follow us) are public, it's normal (in the mastoverse) that we can comment and send just to that person or server. If mastodon also sends to others, that's not our problem, we're not the ones who do it.
Example: Leo comments on a public post by a person I don't know, I see his comment and also the conversation that started the comment, I write my comment which arrives at Leo's server (mastodon), and it is the server of Leo distributing my comment to others, including the person who wrote the main message.
Ps : Just did a test 👆👆👆 and.. Yes, in my delivery report there are just my contacts, so if mastodon or similar sended my comments to a server that doesn't know mine is a mastodon problem not mine.