That’s the federated aspect of those platforms at work. Assuming both instances of a given platform on the Fediverse (here, Fosstodon and kbin.social) are federated together, which they are by default, content from both will be accessible from users of the instances.
How are they able to post from their mastadon account? Do you link your mastadon account to kbin somehow?
That’s the federated aspect of those platforms at work. Assuming both instances of a given platform on the Fediverse (here, Fosstodon and kbin.social) are federated together, which they are by default, content from both will be accessible from users of the instances.