Yeah, anytime someone says "once in a million years" and isn't an astronomer I would doubt those numbers. My money would be on "the number we need so people don't freak out".

On the other hand, it's also easy to overstate the magnitude of a disaster. To the best of my knowledge, no one actually died of radiation exposure from the accident (and probably no one will at this point). There are expected to be a number of deaths (somewhere between 15 and 1100) from increased cancer rates in the larger population. I would hope these weren't included in the "one in a million" claim.