Even if this is true, don't forget that in WhatsApp you don't get the message history when you join a group. Plus, everybody sees that someone joined. That doesn't take the problem away, but it makes the impact much smaller than it sounds.
When someone joins/leaves the group every participant generates and distribute a new key. So as a rogue participant, the keys you’ll receive won’t allow you to decrypt past messages.
PS: this article was posted before the speaker was even done with his talk. This is funny?
However, it's clear that the severity will depend on the particular group. A group of three friends will certainly notice when someone is added to the membership. A larger group of people who don't know each other personally but trust the administrator to administer membership will have to rely on the admin both noticing and taking action, and he may be hampered in doing so by the attacker.
The article states that admins can suppress messages, including the ones that report that someone has joined.
...a WhatsApp spokesperson confirmed the researchers' findings, but emphasized that no one can secretly add a new member to a group—a notification does go through that a new, unknown member has joined the group.