The concern is not the act of reading, but rather the additional exposure to light suppressing melatonin production.

Melatonin plays an important role in the sleep/wake cycle.

Avoiding unnecessary light for a period of 3 hours prior to sleep gives the body a chance to run it's shutdown scripts by producing enough melatonin, etc.

https://www.foundmyfitness.com/episodes/melatonin-insulin-re...