Having worked with lemmy_server code for months, I wouldn’t trust it in practice. Unless someone produces a pretty through test to validate that it actually works as intended when out-of-band data delivery attempts are made. And for discussion sake, I am assuming comment and posting “messages”, and not private messages… which is a whole different set of behaviors.
when a user homed on lemm.ee comments on a post where the community is hosted on lemmy.world, it would get delivered to lemmy.ml by lemmy.world, not the origin lemm.ee instance.
Messages are sent with a digital signature that only the original instance could craft.
I don’t think the actual message carries a signature from the origin instance, in this case lemm.ee - I could be wrong, because it may not unpack and repackage it before forwarding to subscribed instance servers. But there are some very short timeouts on these signatures and HTTP connections, and I think it does unpackage it and repackage a public comment message.
I think lemmy.ml would need to go fetch the profile for the then unknown user to be able to get the display name and other details of the lemm.ee comment creator to display on the comment. The signature for the comment I think would actually only validate lemmy.world to lemmy.ml - and if lemmy.ml already has the profile of the lemm.ee user stored locally it may not verify it (and could be an entirely different install with the same username). But I don’t think many have ventured into study the code in these areas. The whole process hasn’t been examined that much and I wouldn’t be surprised at all if there are issues with post and comment getting dropped because servers aren’t all reachable at the right times for these steps.