You're right -- I very much doubt that even a tiny percentage of reddit's users know that they're using Markdown (it's mentioned as an aside in the "formatting help" section). Plus, of course, it's only a limited subset of Markdown allowed there.

Nonetheless, a tiny percentage of reddit users who know how to use the formatting still likely represents a majority of the set of people who know about Markdown formatting.

I find it infuriating on reddit. The fact it follows markdown indentation rules is completely unintuitive to someone who hasn't read the markdown specification. Given how little people actually want to do complicated formatting I think markdown is actually the wrong choice for reddit.