The country has no import and export goods. The country is already in development will develop more slowly than ever because the item could not be sent to scoop up a pile of benefits from abroad.
Countries have import and export of goods. Poorer countries, will develop more slowly than before because you can not export to seize the benefits from abroad.
The country has no import and export goods. Underdeveloped countries already developed much slower because it can't be exportable to grab the same benefits from abroad.