There's easily a chance Dracthyr/Evoker were not in the pipeline/confirmed at that moment in time. Dragonflight was only in its conceptual stage at that point. As soon as they committed to it, they probably then got to the Dracthyr. Tinker may have been scrapped/postponed for a number of reasons. There's story/info here that are just too close to the truth for it to be an outsider's lucky guess.