@_elena first important thing to know is non of this has to do with activity pub. (i say this bc from the last article i got the impression that this was possibly an assumption.)
Applications talk to each other through an interface. Interfaces between applications are generally called API's.
Mastodon created their own API to communicate with clients (eg web browser, smartphone apps...). Others have implemented a similar API so that Mastodon apps could also work with them. Pleroma and Akkoma both have this, I'm pretty sure Pixelfed also has this, possibly Firendica too.
That's why it works, the Mastodon app sends requests, the server understands them and provides a proper reply. Misskey never did that. Sharkey was forked from Misskey, so unless they added that, it won't work because Sharkey doesn't understand the requests that the Mastodon app is making.
Applications talk to each other through an interface. Interfaces between applications are generally called API's.
Mastodon created their own API to communicate with clients (eg web browser, smartphone apps...). Others have implemented a similar API so that Mastodon apps could also work with them. Pleroma and Akkoma both have this, I'm pretty sure Pixelfed also has this, possibly Firendica too.
That's why it works, the Mastodon app sends requests, the server understands them and provides a proper reply. Misskey never did that. Sharkey was forked from Misskey, so unless they added that, it won't work because Sharkey doesn't understand the requests that the Mastodon app is making.