I'm going to play devil's advocate for a moment.

following best practices we laid out in our internal documentation

Are you absolutely sure those "best practices" are relevant or meaningful?

I once worked with a junior dev who only cared about "best practices" because it was a quickly whipped document they hastily put together that only specified stuff like coding styles and if spaces should appear before or after things. That junior dev proceeded to cite their own "best practices" doc with an almost religious fervor in everyone else's pull requests. That stopped the very moment I made available a linter to the project, but mind you the junior dev refused to run it.

What's the actual purpose of your "best practices" doc? Does it add any value whatsoever? Or is it just fuel for grandstanding and petty office politics?

his code works mind you,

Sounds like the senior dev is doing the job he was paid to do. Are you doing the same?

It’s weird because I literally went through most of the same training in company with him on best practices and TDD, but he just seems to ignore it.

Perhaps his job is to deliver value instead of wasting time with nonsense that serves no purpose. What do you think?