I can see why we're splitting hairs here regarding full extent of the sundering, but I think the game has been pretty clear that Hydaelyn split "the star" which I interpreted to be the planet into 13 more dimensions and the dragons have said numerous times they showed up after the sundering and made a pact with Hydaelyn to protect the star.

Where I read into this discussion, is the amount of power the dragons have are extraordinarily great, so without an actual lore official source saying otherwise, I'd assume the dragons are not sundered because they were on another planet that wasn't sundered. This would fall in line with their power level and the Ascian machinations around their eyes in Stormblood as a key resource to accelerate the war in Ala Mhigo as well as motivating the church in Ishgard to also accelerate the Dragonsong War.