Can confirm the key activates in Japan. You might want to remove the conflicting notice that states "Please note that this product is region locked and can't be activated from Japan."

Thanks for the reply.
The region lock for Japan has been removed.
I was able to purchase and activate it!

皆さんへ　現在ここで購入したら日本からでも有効化できます！
こちらで購入し、確認しました。