I don’t think there’s a need for a GUID, in fact it would be quite difficult - every instance would have to check with every other instance to ensure that the ID’s are unique. Meanwhile, if we just have the federated host picking a number, then every other host uses that number followed by @hostinstance, we don’t need cross-checking but still have unique ID’s for everything.
For example, https://lemm.ee/comment/123456 would be a different comment to https://lemmy.nz/comment/123456 (as it is currently also), but the first comment could be found on the 2nd instance as https://lemmy.nz/comment/[email protected].