Not true. Changes to the OS, particularly to the scheduler, can affect CPU-bound work a great deal. It's "your code", but the OS decides when and where it runs. For example, the changes between Linux 6.5 and Linux 6.6 led to a >20% uplift in TensorFlow and some smaller uplifts to Blender. This is the same software on the same hardware.

https://www.phoronix.com/review/linux66-epyc-xeon/3

I couldn't track down a more detailed scheduler-specific benchmark. I remember reading one on Phoronix a few months back...