If any publisher (in this case, a lemmy instance) does not require the author to consciously consent to assigning the copyright of the comments to the publisher or some other entity, then by default the copyright of the comment is retained by the author who is allowed to write literally whatever licenses they like and as many licenses as they like for however many people they want.
Tacking a license to the end of a comment posted to just about any website does not actually change how the sites content is licensed.
If any publisher (in this case, a lemmy instance) does not require the author to consciously consent to assigning the copyright of the comments to the publisher or some other entity, then by default the copyright of the comment is retained by the author who is allowed to write literally whatever licenses they like and as many licenses as they like for however many people they want.
https://gizmodo.com/who-actually-owns-your-content-when-you-post-it-to-the-1819953868