This is not true, Lemmy can definitely have plugins and there is an extensive discussion about this topic. The conclusion is that plugins should be implemented in webassembly, so that they can be written in many different languages. See extism for details. Whats needed is someone with a clear use case who can implement a proof of concept, as it wouldnt make sense to add plugin hooks that no one uses.
Also mod tools can be implemented as api clients such as LemmyAutomod.