(Kudos on doing this in Racket. Besides being a great language to learn and use, using Racket (or another Scheme, or other less-popular language) is a sign that the work comes from genuine interest, not (potentially) just pursuit of keyword employability.)

Side note on Lisp formatting: The author is doing a mix of idiomatic cuddling of parenthesis, but also some more curly-brace-like formatting, and then a cuddling of a trailing small term such that it doesn't line up vertically (like people sometimes do in other languages, like, e.g., a numeric constant after a multi-line closure argument in a timer or event handler registration).

One thing some Lisp people like about the syntax is that parts of complex expression syntax can line up vertically, to expose the structure.

For example, here, you can clearly see that the `min` is between 255 and this big other expression:

    (define luminance
      (min (exact-round (+ (* 0.2126 (bytes-ref pixels-vec (+ pixel-pos 1)))   ; red
                           (* 0.7152 (bytes-ref pixels-vec (+ pixel-pos 2)))   ; green
                           (* 0.0722 (bytes-ref pixels-vec (+ pixel-pos 3))))) ; blue
           255))

Or, if you're running out of horizontal space, you might do this:

    (define luminance
      (min (exact-round
            (+ (* 0.2126 (bytes-ref pixels-vec (+ pixel-pos 1)))   ; red
               (* 0.7152 (bytes-ref pixels-vec (+ pixel-pos 2)))   ; green
               (* 0.0722 (bytes-ref pixels-vec (+ pixel-pos 3))))) ; blue
           255)))

Or you might decide those comments should be language, and do this:

    (define luminance
      (let ((red   (bytes-ref pixels-vec (+ pixel-pos 1)))
            (green (bytes-ref pixels-vec (+ pixel-pos 2)))
            (blue  (bytes-ref pixels-vec (+ pixel-pos 3))))
        (min (exact-round (+ (* red   0.2126)
                             (* green 0.7152)
                             (* blue  0.0722)))
             255)))

One of my teachers would still call those constants "magic numbers", even when their purpose is obvious in this very restricted context, and insist that you bind them to names in the language. Left as an exercise to the reader.