Mastodon’s WIP implementation uses the conventions of Smithereen semi-standardized as FEP-400e. It’s not something “incompatible with any other platform” (it’s not commonly implemented, but it’s not bespoke for no reason either). FEP-1b12 used by Lemmy also has it’s own quirks (why are we Announceing activities?) and the specific implementation used by Lemmy will likely not interoperate far without breaking changes that will upset one or the other party, mainly the fact that users and communities can use the same webfinger handle.
Either every single other software needs to have specific quirks for Lemmy in order to handle this as most (reasonably) assume the username@domain combo will be globally unique, or Lemmy instances need to go on an account or community deletion spree to make this non-unique. You can easily DoS a user or community’s federation outside the threadiverse bubble by setting up one of the other with the same username on the same instance.
PS: Lemmy is the only platform that has had an exploit that overshadows Mastodon’s success
what? I’d like to remind you Lemmy is the third most largest open AP server software. After Mastodon, and Misskey. (by a not insignificant margin, with Misskey having 8.6% of all known users by FediDB, and Lemmy having 3.8%) Just because they’re Japanese doesn’t make them any less a part of the network, and they do have their own innovations (MFM, and emoji reactions are just the ones that federate. They also had quotes before many others but I think they’re not the first on that one).