That is only if you use the debian-specific definition of "stable" which is "does not change". The rest of the world thinks of "less bugs" when they think of stable software.
Back here in reality, rolling release is less stable because more bugs in the software get through. And this is a reasonable expectation and not some magical fairyland where bugs never get written so being right up against the dev branch is as stable as being on the stable branch.
We really live in two different software worlds. Every software I'm using has its number of bugs a purely decreasing function of time, especially in the "main" paths and use cases.
If it were true, it means there wouldn't be bugs in the first place because they wouldn't have gotten written. The very fact that the bugs got written implies new bugs can, and will, be introduced.