It sounds like they've been running an integration test which uploads an image once per minute for pretty much their entire existence (they're a little over 6 years old.)
This is sensible. It's the only way to get instant feedback when the site stops working for any reason.
The sampling was done only for the old-style 5 character URLs. They've since moved to 7 character URLs. Depending on how that 3.2 million number was reached, you maybe should be calculating rate using just the time they were at 5 character URLs. I don't know when the switch occurred though.
When you get big enough monitoring and integration testing start to look pretty similar. The most reliable kind of monitoring is the kind that actually uses your system, end-to-end.
This is sensible. It's the only way to get instant feedback when the site stops working for any reason.