Think about it like this: even when you link other posts in lemmy, you link them in their home instance, because there is no way to link posts so that everyone gets one to their own instance as you can do with communities in the threadiverse. Neither can you repost it in any meaningful way, since that just means copying the content, which would make it appear as though you said it yourself.
Since mastodon and lemmy are federated, could one have postet the mastodon toot directly?
Think about it like this: even when you link other posts in lemmy, you link them in their home instance, because there is no way to link posts so that everyone gets one to their own instance as you can do with communities in the threadiverse. Neither can you repost it in any meaningful way, since that just means copying the content, which would make it appear as though you said it yourself.