Earlier quoted context omitted.
"The state" refers to the governing apparatus, not a particular state within the union. I understand there may be some confusion if you are not familiar with the US government and typical ways of talking about it.
https://constitution.congress.gov/constitution/amendment-10/ Please cite me where the power to enact a requirement to wear masks is explicitly granted to the federal government. I'll wait.
When Trump was given his presidential megaphone he was able to convince a LOT of people to not wear masks by simply not wearing one and then telling people that's fine. At least by taking away the megaphone that's a step in the right direction. Even if there's no specific law that backs him up, that alone may save lives.
Second, even if technically not legal, presidential orders are OFTEN followed. It can be challenged in the courts for years but in the mean time, an order like that will probably have the effect of getting at least another 50M masks put on. That's a fucking win.
The other thing that is 100% legal, is the executive branch can restrict funding in specific ways if a state does not behave. If I was Biden, I would start there.