That is correct, it is impossible for the federal government to enforce a standard.

Harris, fumbling at least three times over a sales pitch of a federal mandate that might not actually be a mandate, should have simply said: “We will listen to what the health experts say at that time. Particularly not being a doctor, anything else that I might spontaneously conjecture on in this interview would be speculative, and not particularly informed. But, if the health experts strongly advise that a federal mandate be imposed, then we will consider it. We are considering all options to protect the American people.”
https://www.cnn.com/2020/09/06/politics/kamala-harris-two-justice-systems-cnntv/index.html