This property is documented to be a boolean that defaults to true.  Instead, it defaults to off and doesn't check the value passed when being set, so setting it to either true or false does the same thing, turning the behavior on.

From libsoup/soup-session.c, set_property:

        case PROP_SSL_USE_SYSTEM_CA_FILE:
                if (priv->tlsdb) {
                        g_object_unref (priv->tlsdb);
                        priv->tlsdb = NULL;
                }
                g_free (priv->ssl_ca_file);
                priv->ssl_ca_file = NULL;

                priv->tlsdb = g_tls_backend_get_default_database (g_tls_backend_get_default ());
                break;

(The "value" parameter isn't referenced at all)