Snowpoint or snowpeak? Apparently you're confusing sinnoh with zelda twilight princess.
According to picross, ash-greninja functions just like a mega would: cannot run regular greninja and ash-greninja on the same party, much like you can't run, say, regular charizard and mega charizard-x simultaneously. But you can run ash-greninja alongside mega charizard-x, which would be contrary to the usual "one mega at a time" rule if it was a mega. Similarly, shuffle has an ash-greninja sprite as well, but it's not officially in the game (as of v.1.3.x) and there's no mega stone-like item for greninja in the game either; it's its own separate icon, like the "pikachu emoji" ones, as the hackers confirmed. So it's certainly not treated as a mega there either.