Basically, such tools, if and when they reach the wrong people, can be used by total unskilled people to find and exploit security issues which were previously missed by multiple security audits.
They give specific examples from OpenBSD and Linux, including chaining of security issues to gain privilege escalation on Linux.
It's not as if exploit toolkits and paying shady entities for zero-days didn't exist before. Bugs happen, stay on top of it, there's nothing else you can do anyways.
The issue is that this tool takes bug hunting and exploitation to a whole new industrial level that doesn't require much expertise. Any “script kiddy” can use it if they get their hands on it.
Other AI tools demonstrated ability to disassemble machine code and ability to comprehend ancient code (e.g. COBOL) to find security bugs in it and write exploits for them.
I've been in this industry for four decades now, this is a new and scary level.