He's not using a repeater, he's using a keyboard synchronizer - this one specifically: https://www.amazon.com/gp/product/B07NZSZFL8/. There's no reason to use a repeater at all - those have always been against ToS.

Now some people don't agree with using a hardware broadcaster at all. Blizzard said they're banning input broadcasting software - so some have gone for input broadcasting hardware. But there are also solutions that don't broadcast anything, and instead use mouseover to change window focus and/or keyboard commands to change window focus - all of which can be macro'd for a single input.