He's not more productive, but even if he were, it wouldn't affect his getting fired.

Productivity isn't everything.

Why?

Uploading code to ChatGPT can be done by trainees.

Yeah, but the code coming out of ChatGPT is generally not in a state that you want to commit straight into you repo. Making adjustments (and writing the original prompt) is where your expertise comes in.

You can just submit the trash code and let a senior fix it for you in code review.