Maybe not even that. Lemmy is released under the AGPL3. This means that modified versions of Lemmy have to also be released as free software under the AGPL3 or a compatible license. To release a derivative work under an incompatible license you would need to own the code or be given permission by each contributor to do so. For any contribution where you can’t make a deal with the author, you would have to rip it out of the codebase entirely. Note that this is true for lemmy devs as well. If there is no Contributor License Agreement that states otherwise, they cannot distribute the work of other contributors under an AGPL3-incompatible license.