Given the list of animals that have, or may <
https://en.wikipedia.org/wiki/Mirror_test#Mammals> why would it be so astounding to add a dog to that list?
(NB, I don't know where I saw it but if you look at <https://en.wikipedia.org/wiki/Mirror_test#Mammals_3> about pigs: "Pigs can use visual information seen in a mirror to find food" - someone recently found dogs doing the same; finding hidden food with mirrors with distinctly better than chance).