I don't know about mystery snails, but I've had pond, ramshorns, MTS, and mini-ramshorns develop pretty big populations during the cycling process.
I think snails (at least those...) are pretty tolerant of crappy water conditions.
And, what are you using to cycle your tank, I didn't see that mentioned anywhere.
I think it's very unlikely to establish a sufficient bacterial colony in that amount of time, usually it takes at least a couple days to get the ammonia processing ones established, and then quite a bit longer to get the slower-growing nitrite processing ones established.
The only other things I can think of is if you used some media that was already established, used some sort of bacteria-in-a-bottle that worked exceptionally well, or you have some plants sucking up the ammonia (but then you wouldn't see nitrates...)