Does Snapshot take place on the Reckoners' Earth, or one of the other Core Possibilities? (Based on the reading you did I would assume the latter, but it doesn't hurt to get confirmation on that).
It's one of the Other Core Possibilities.
The idea of going meta-series with Reckoners is to offer me the chance to play with quantum possibilities and alternate dimensions, which is something to cosmere's not set up to do. We'll see how big this one gets, but I'm fond of silver age comics, and the idea of alternate realities they explored. (Often in goofy ways because...well, silver age.) This also gives me a realm to do some magic, like the Epic powers, that I can hand-wave a little more, rather than confining everything to the structure of the cosmere.