Note that this only applies to laws or refs passed after 1/19/2021; everything passed before that by the Feds is thus deemed OK by state.. including the entirety of the 1934 NFA.
It actually says:
"Subsection (b) does not apply to a contract or agreement to provide assistance in the enforcement of a federal statute, order, rule, or regulation in effect on January 19, 2021."
It's not a federal law it was speaking of, but any "contract or agreement to provide assistance" in enforcing said law. So, no it would be illegal for local LE to assist the feds in enforcing any federal gun law unless an agreement was previously made between local LE and the feds to provide assistance in a particular instance, and this agreement was still in effect on January 19, 2021.
So, if your local PD shows up at your door with the feds after 9/1/2021 to enforce a federal gun law, and they weren't under agreement to assist them with your case back on Jan 19, then your local PD is breaking this law.