Ok, so I'm mildly disappointed that my question didn't get a response but I found the answer myself.
I made an error by omitting the config variable in rwmb_meta, which should have been:
rwmb_meta( 'company_password' , array() , $postid )
but it still didn't work with the code provided in the docs.
Changed it to:
if ( wp_check_password( $password , rwmb_meta( 'company_password' , array() , $postid ) ) ) {
// code when password checks
}
Please correct in the docs, I'm assuming it has never worked as described.