Glad you like that article about What Kids Don't Learn, when they don't have appropriate challenge. smile It is a favorite of mine.
One word of caution: When discussing appropriate challenge, be prepared to define challenge as being consistent with:
- the Federal definition of gifted (as found on the NAGC website),
- teaching in the ZPD.

I mention this because from time to time some have used the term "challenge" to mean:
- not focusing on developing a child's strengths,
- focusing on a child's relative weaknesses,
- nit-picking to find faults, possibly encouraging perfectionism and/or making kids risk-averse and/or inclined to avoid stretching their skills and abilities. Essentially making kids afraid to be "wrong"; Instilling a fixed mindset instead of a growth mindset.

Another article which may be of interest to you, and also the OP: A report from 1997, titled What it Means to Teach Gifted Learners Well, by Carol Ann Tomlinson, Ed.D., University of Virginia. This report is on the website of the National Association for Gifted Children (NAGC).