Instead of sending the entire object embedded in the activity the secure way would be to only the URI instead. This is permitted by JSON-LD.
In the receiving side, if the object is untrusted (i.e. if it isn’t signed or if it’s from a separate authority from the parent object containing it) it should be thrown away and the id should be fetched from the remote instance directly (same as it would happen if it was a URI instead of an inline object). This is completely an oversight on Lemmy’s implementation and not a protocol problem.