Pressing a key to trigger an action that you will then send additional input to is an entirely different sequence of events than whack-a-mole, where you are definitionally not triggering the events you need to respond to.

I'm not talking about latency (though I don't fully agree with your statement but I've covered that elsewhere). I'm talking about the GP's comparison of reactions vs musicians listening to unquantised pieces.

You simply cannot use musicians as proof that people have these superhuman reaction times.

But here we're talking about not being able to notice whether calc.exe opens in less than 300 milliseconds, not how fast we can react to it opening? It's the same thing with audio latency (and extremely infuriating when you're used to fast software where you can just start typing directly just after opening it without having to insert a pause to cater to slowness)

No it's not the same thing with music latency. For one thing, music is an audio event where as UI is a visual event. We know that music and audio stimuli operate differently.

And for the music latency, you can here where the latency happens in relation to the rest of the music piece (be the rock music, techno, or whatever style of music). You have a point of reference. This makes latency less of a reaction event and more of a placement event. ie you're not just reacting to the latency, you're noticing the offset with the rest of the music. And that adds significant context to perception.

This is also ignores the point that musicians have to train themselves to hear this offset. It's like any advanced skill from a golf swing to writing code: it takes practice to get good at it.

So it's not the same. I can understand why people think it might be. But when you actually investigate this properly, you can see why DJs and musicians appear to have supernatural senses vs regular reaction times. It's because they're not actually all that equivalent.