TFA says they need to send the target a text message.
The exploit must be something like a buffer overflow in iMessage. Which we know bugs like this have been fixed. Remember the text of death which could crash any iPhone from a couple years ago?
Are you thinking of the "Stagefright" bug that did RCE via SMS on Android devices? Or maybe the Chinese censorship code that crashed iOS when people sent the Taiwanese flag emoji? 🇹🇼
"Bugs like this have been fixed" != "all bugs like this have been fixed". That is, some similar bugs having been fixed does not make such an exploit impossible.