I would say that I would expect a genuinely gifted child to be able to find constructive ways of occupying themselves at nursery, or in a properly organised Reception class UNLESS the nursery or reception class was deliberately putting a ceiling in what the child could do.
If you have a child who can read, but can't occupy themselves with a book or maths equipment or toys or pen and paper or any other of the myriad pieces of equipment available in a nursery / reception class, then I would say that their ability is currently rather narrow, and everyone involved could do with looking at that education in the round. What is it that is stopping that child learning through play with the equipment, resources and materials around them? Is it that they have come to regard 'education' as 'only reading / writing', because that has been prioritised / praised over other forms of learning, by parents or teachers? Do they in fact have few inner resources and require adult attention?
A child who can read has the world at their fingertips. Finding out why they are bored, in a well-equipped nursery, would seem to me to be a higher priority than worrying about reception reading.
My DS, a fluent reader on arrival in reception, LOVED Reception and its well-stocked bookcase, short taught sessions of phonics (which he learned from for writing) and other subjects, and large swathes of time for exploratory, imaginative play (which for him did involve a LOT of reading of books for older children, and a LOT of complex maths involving negative numbers). Year 1, with its more rigid structure and limited-task worksheets, was MUCH more of an issue, despite it looking much more 'educational' in its narrowest sense.