you should see the reaction time of the fly that's been in my house for days now

Yes, smaller animals, faster heartrate, more direct wiring, they probably experience time different than we do.

Many human responses are purely unconcious muscle memory due to nervous system latency, this also implies the brain has evolved to be highly predictive as to compensate.

I would go one step farther: I bet that plenty of humans could pass the test that the birds were subjected to at 120Hz or even higher. They didn't test whether the birds could actually resolve stimuli at high temporal resolution -- they tested whether the birds could detect flicker. A bright source pulsed at 120Hz is easily perceptible (and incredibly annoying), especially if it's a square wave with a duty cycle that isn't especially high.

Where are the control animals?

Watch a falcon pluck a drone out of the air, inserting its talons between the blades. They see everything faster.

This is an inherent limitation in trying to use Hz to measure a system which does not really have a concept of frequency at all. For instance when you have a motor system -> vision feedback loop, humans have been shown to accurately discern latency differences of as low as 1ms in some studies. Though if you've ever played on a first person game at 60Hz, then 120Hz, then finally 144Hz and beyond, this probably won't surprise you. It's quite strange how you can perceive multiple degrees of "instantaneous".

It's also been shown that latency differences as low as that don't really have any noticeable impact on human performance though, so it's likely we can merely perceive that to allow our brain to subconsciously fine-tune our motor system. You'd be a very clumsy human if your motor system only had a resolution of ~20ms throughout. Despite it obviously being necessary to help you learn to use your motor system, we don't really seem to get to use that high "resolution" much consciously.

Also I might be comparing apples to oranges here, because you could also argue that a camera taking one picture every 10s could discern differences as low as that, if you take the pictures at the right time. But we also don't work like cameras, which brings us back to the topic at hand of frequency not being a good metric since our vision is more of a continuously operating system.

Make of that what you will.

Flicker fusion studies typically control for this motion-based detection artifact by using head-fixed preparations or tracking eye movements, which the authors here attempted by using stationary perches and monitoring head movements during trials.