I don't think this book is opinion, so much as, a compilation of information on a small selection of gifted children
It IS that, for sure, and that's quite interesting in the way that reading these boards is addictively interesting. However, I think th "...and this is the level these kids are at, factually" part is opinion.
Her IQ may be an underestimate, true. She took a short test (the RIAS) and did just average on 1 out of the 4 subtests, which brought her score down. However, I think the other subtests are likely to be accurate, and they were a pretty even profile on the high end of MG.

She IS, however, a very driven and "schooly" child.