You’re overthinking this. The point of Broom Boy is to show how the Force is for everyone and that you don’t need to be mentioned in a prophecy or part of a certain family to use it. And in order to establish this in a quick and subtle way, they showed him grabbing the broom. Showing his enhanced reflexes would require a way longer scene, putting too much emphasis on him. And simply mentioning his abilities in dialogue would be lazy and too on the nose.
I don’t agree with this for two reasons:
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We’ve seen Rey almost instantly gaining Force powers for two films, a clear indication that the rules with regards to Force powers have changed radically.
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The scene with broom boy might have worked better, if he had been shown trying a Force pull alone in his room after the scene where the children reenact Luke trolling the FO. He could have been shown concentrating, failing, concentrating again, and then be stunned, that it actually worked. That could have been a short scene. However, in TLJ the boy does it so casually, it’s second nature to him. This doesn’t jive with what’s been established in the pre-Disney era.