"diff to ..." is an "add to review as" method. it doesnt work, because it uses the revision that you give it as the from revision for all the files that you add. since all the from revisions will almost definitely have different ids / numbers / hashes / whatever, this could never work, at least not for a whole changeset.

the only way that it could work is if you only wanted to add a single file. and we dont enforce this.