Hyperlexia was thrown around a lot when DS was young as well. He was never officially diagnosed, which is good as he turned out to be an early reader instead. I did meet a child a few years later with the hyperlexia diagnosis. So based off my own child and 1 example (meaning take with a grain of salt)...
A hyperlexia child will spend a lot of time with words and letters at a young age. One of the first steps to reading is decoding, which a hyperlexia child will excel, as will an early reader. My understanding is that a hyperlexia child does not really go beyond that with the reading whereas an early reader will be able to create own stories, write own books, create new words, and incorporate "reading" into various formats of play. It is an ability to be plastic with the skill and transfer the skill to various situations. The hyperlexia child (again, n=1) I met could not do that. The child could clearly read, but could not really do anything with it.
Hopefully, someone with actual knowledge will post soon.
Thank you for responding. What was your experience with Cogat or WISC scores? Did they change over the years as other children caught up in terms of reading ability?
