Well, the paper was received from Nature on 05 July 2017, so it isn't surprising that they didn't cite the work from Cueva and Wei published 21 Mar 2018.
That's not quite how the process works. These papers go through multiple (> 2) revisions. At any iteration, there's ample opportunity for updating references. This applies double given how long the Cueva/Wei work has been available (preprint & CCN'17 (?) contribution).
If you are making that argument, you should refer to the ICLR deadline, which is well earlier, but still after July 2017.