To be fair, it seems that the way it works behind the scenes is using SSO and OIDC (see https://docs.activitypods.org/app-framework/backend/application-registration/ ) so applications would need to support that before.

Lemmy only added this three weeks ago, https://github.com/LemmyNet/lemmy/issues/2930 and pixelfed still doesn’t support it directly as per https://github.com/pixelfed/ideas/issues/14

Likely the reason they went with Mastodon is because the support for the above is more mature there, but seeing as this is ActivityPub-based I think in theory it should work with Lemmy et al. (Doesn’t rule out there being app-specific bugs and such that need to be ironed out, but that’s often the case for new releases regardless.)

That said I’m having a bit of a hard time following the docs. If I’m reading https://docs.activitypods.org/architecture/authentication/ right you may not be able to use a lemmy.world account as your credentials - it seems like there’s some configuration on the “application” end to enable this, meaning the lemmy.world admins would have to turn it on for this to work. (But if you had it turned on and successfully working elsewhere and then posted as that other account to lemmy.world, it would federate as usual.)

That’s not true it’s compatible with anything running ActivityPub. It currently has build a Mastodon client as a demo that’s why right now you can use it with Mastodon. They built Mastopod in a week. Part of the pitch it showing devs how relatively simple it is to build apps on top of it. So if someone builds a Pixelfed, Lemmy etc app then you absolutely can use it