There is substantial Supreme Court precedent regarding this, Americans have a Constitutional right to freedom of travel on par with freedom of speech. Exceptions are extremely narrow and limited. I don't think people here realize how many times this has been reviewed by the court in cases the government deemed exceptional. This is not new territory legally.
The interstate commerce clause is fundamentally restricted in scope such that it does not violate fundamental individual rights i.e. you cannot use the commerce clause to restrict free speech.
Yes, it was terrible precedent but it also has no relevance here. If you could abrogate fundamental rights with the commerce clause then those rights effectively don't exist and the Supreme Court has consistently ruled on this. You can't use the commerce clause to infringe on the right of free speech, the right to bear arms, etc. This has been argued in the court many times and the outcomes have been one-sided in favor of the individual right.
Freedom of travel -- what we are talking about with a "lockdown" -- is a thoroughly adjudicated fundamental right per the Supreme Court. Using the commerce clause to restrict travel in various ways has already been rejected by the Supreme Court, repeatedly.
The interstate commerce clause is fundamentally restricted in scope such that it does not violate fundamental individual rights i.e. you cannot use the commerce clause to restrict free speech.