An LED is still a photodiode, as this blogpost noted. But you'll get more consistent results with a real photodiode (or phototransistor, if you want a bit of amplification added in)
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Obviously the diode works here and for this project. But given higher levels of sophisticated photosiodes (but still a similar circuit), you can choose photosiodes that detect infrared light
We humans light up like a light bulb in the infrared spectrum. So two IR Photodiodes is all you need for a human motion sensor.
A motion sensor detects motion by looking at two pixels of IR light. Motion is 'defined' as one pixel lighting up.... Followed by the second pixel lighting up. If only one pixel lights up, then no motion (it's just a glitch). Real motion should show up across both pixels in a slight time delay fashion.
IR Photodiodes (or phototransistors) are used because it's the spectrum of light humans shine the most brightly in, despite being invisible to us humans it's the best sensor at the electronics level for these purposes.
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There really isn't much electrical complexity to the motion sensor that this blogpost was trying to debug.
I would suggest building your own motion sensor as the next step in your EE journey.