That's not what the study says. from the abstract:

> We conclude that while context files are useful for specifying non-standard coding practices, any attempts to improve performance should be rigorously evaluated before deployment.

The purpose of AGENTS.md is not to improve "coding performance" as the study looked at, it's to give an agent practical instructions that are useful to your specific workflow. For example you want it to use a certain format or specific tools for your project. This is stuff that can't be learned during training and must be loaded into the agent's context at the project level.