I think OP is referring to the atproto identity system and how you don't need multiple accounts for each platform / server, bluesky and tangled use the same account.
Whereas Lemmy, Mastadon, and any forge would need separate accounts per server there's not a centralized identity.
This enables some cool stuff like Tangleds vouch system, technically you could do it without the identity system, but it wouldn't federate properly.