Keep in mind that Apple’s modern SIM lock implementation is built into the OS and has nothing to do with the modem being locked, so all assumptions about modem locks no longer apply.

Apple’s SIM lock “feature” checks in with Apple over the internet (including over the disallowed SIM, since the modem itself is always unlocked) and refuses to let you proceed if the current SIM doesn’t match the policy returned by the backend.

Obviously the policy can be changed at any time. This is also how this “flex carrier lock” is implemented.