I want to read from and post to #Lemmy communities from my #Mastodon account. I need to register on every Lemmy instance of interest? I know #ActivityPub is flexible and Mastodon uses a different "profile" of it than Lemmy. But I can't post a basic note to Lemmy from Mastodon? What use is ActivityPub, given that this situation is no better than posting from Mastodon to Wordpress?

It doesn't seem to count that #Hubzilla/#Zot got all this right ages ago with #OpenWebAuth. If Lemmy instances and Wordpress instances and Mastodon instances were Hubzilla/#Zap instances, it would just work.