There is a wholy unsupported assumption that a US reactor of completely different design would failure in a manner comparable to Cherynobl.
Where did you get that? The article never said or implied that the Californian reactors would fail in the same way.
The author directly compared the radiation cloud of Cherynobl over Europe with the size of the US, not to give the Cherynobl accident a sense of scale, but following the discussion of the CA reactors to imply the same could happen.
Clearly the article said that there could be a massive, accidental release of radiation from the California reactors. But that doesn't mean the way they failed would be the same as the way Chernobyl failed. Similar result does not imply similar means of failure.