Currently in bigbluebutton_proxy.php, BBB API guest parameter is being set as either true or false see https://github.com/moodle/moodle/blob/main/mod/bigbluebuttonbn/classes/local/proxy/bigbluebutton_proxy.php#L110

We shouldn't be determining this in the proxy, but instead pass this value from the method as a parameter, like
$data['guest'] = $guest;