The point is that even if all code commits are scanned as safe by ai, black hats can still analyse the commits and diffs to find vulnerabilites for people who havent patched yet.

Scanning every commit doesnt automatically make everyone in the world patch immediately, vulns can still be found from commits and diffs and used against those who havent patched yet.

I believe their point was that:

"How likely is this diff a patch for an existing vulnerability?"

Seems to be an easier question to answer than

"Are there any new vulnerabilities introduced by this diff?"

In other words identifying that a patch is for a vulnerability is typically easier than finding the vulnerability in the first place.

The diff yields the patched code which is used to produce the exploit.