Why is my drivetrain so noisy after replacing the chain?
Fixed gears don't have as much tolerance for mismatched wear patterns as geared bikes, because there is no "give" in the drive train system.

The small amount of wear shown on the used cog is enough to cause this kind of noise in the case of a fixie. I'd be surprised if the same amount of wear caused you any problems at all on a geared machine.

It's also possible that the chain was over tensioned. I know you said that you use a tensioner, and that you didn't change the settings, but you used a new, unstretched, and therefore shorter chain. You may have needed to relax the tension by a small amount.

As you removed the wheel to change the cog, It may be that the tension has changed as well. Hard to say for certain, as you made the noise go away.