The main helpdesk of Horizon 2020 in Europe can only answer the case of Horizon 2020 participation. Therefore their answer might be “If participating in Horizon 2020 or programmes of EU, all participants and partners must obtain PIC codes”. However, coordinated calls are a special case in which Japanese organisations are technically not participating to Horizon 2020. However, you could obtain a PIC code and leave it as declared. The process to obtain a PIC code is very easy and many Japanese organisation already obtained one. You can use it later to participate in regular Horizon 2020 projects.