Fortunately they added this statement because (IANAL) I believe those developers didn't perform any of the activities listed in the bullet points of the letter.
However all of third party WhatsApp automation is a gray area. Everybody knows that WhatsApp doesn't welcome it (I really don't know why) and they have the money to litigate, especially because no big business will ever build anything on the top of WhatsApp without a previous explicit consent. For example a bot API. I never understood why they didn't implement one. I hoped that after the FB acquisition they would, an exact copy of the Messenger one, but they still didn't. Now that all the founders left maybe things will change but I wonder what's FB's plan. WhatsApp is bigger than Messenger in many countries and with the EU stance against data sharing between the two companies I wouldn't be surprised if the plan is to move all WhatsApp users to Messenger.