Yes, using this bulk WhatsApp tool is legal, as long as you use it responsibly. Avoid using it to spam or deceive people on WhatsApp. If you send messages that cause users to report your account, WhatsApp could still take action against you.
You might have read that WhatsApp does not allow the use of automatic bulk-sending, and this is true. Nevertheless, you can safely use this tool within WhatsApp’s Terms of Service, because this is NOT a one-click automated WhatsApp bulk sender. For it to work, you must press SEND for every recipient added and it will bring you to the official WhatsApp apps to complete the sending. Therefore, it is not considered an automatic bulk-sending process. And it’s still much faster than messaging each person manually.