This is a static site. To reach that sort of bandwidth out of nowhere you'd need to publish the blueprint for a teleportation machine
That said, instead of depending on unreliable heuristics, they should just allow an option to change the behavior. The "current policy" to charge small sites on the free tier thousands of dollars instead of just throttling/shutting down the traffic is really predatory.
1 million downloads (= visits) is nothing for "going viral overnight"
I wager the vast majority of people in the free tier would gladly cap their traffic at the (generous!) bandwidth offered by Netlify. Even to the majority in paid tiers, 100k bills where there previously was none must be unwanted and unintended.
I mean, we all know dark patterns are a thing...