This is indeed expected. I specified in the "Caveats" section in README.md that the exploit does not work on Ubuntu v6.5 (because it enables a certain kernel config value that indirectly mitigates the exploit starting from v6.4), and may not work on other kernels above v6.4 depending on the config.

Ah I read that versions upto v6.6 were affected and didn't notice the caveats. Thanks