From a strictly constitutional perspective, you may be right. In the America we live in, which is one of rapid transit, public health policies differing by State will be utterly ineffective at stopping an actual pandemic.
To be sure, the feds have to date given no evidence they’re any more capable of stopping a pandemic, but at least it’s theoretically possible for them to do so.
So in the final analysis it gets down to the old saw about whether adherence to the Constitution requires the country to commit suicide. Or to the words of Jesus about whether the Sabbath was created for man, or man for the Sabbath.
/johnny