I don't think AirTag work that way. AirTag protocol is specifically designed so Apple or other parties will not be able to track users by serial numbers.
Where there's a will, there's a way. Apple is very clear law enforcement can approach them with any AirTag and they will immediately be able to tie it to a user.
One doesn’t exclude the other - a physical airtag may have an ID available, but not broadcast it anywhere.
Also, “when there’s a will…” doesn’t really apply to cruptography