What's the earliest version of the Wu and Gagnon-Bartsch paper? Arxv shows August 2017 as the most recent version.
WBG is a working paper that was posted in arvix on August 3, 2017. It's perfectly possible for Spiess to be unaware of it until November or December.
By the way, WBG doesn't cite Spiess either at any point in the text. And the paper lacks a list of references at the end.
OP links to the draft one of the authors posted on his own website, December 2016 http://www-personal.umich.edu/~johanngb/working/loop.pdf
This and the arxiv draft both DO have a list of references, after the main text, before the appendix. They could not have cited Spiess since his paper did not exist even when they posted on arxiv.