It is absolutely 100% true, Klinefelter's syndrome is the symptoms of the disease Seminiferous Tubule Dysgenesis. Grab yourself an endocrinology text book and check it out! Or read mine, Williams Textbook of Endocrinology 8th Edition, page 879 "Seminiferous tubule dysgenesis is a common cause of primary hypogonadism, and male infertility. This syndrome was first defined as a clinical entity by Klinefelter and associates. The characteristic features, which first become manifest during adolescence are, gynaecomastia, a variable degree of eunuchoidism, small atrophic testes, with hylanisation of the seminiferous tubules, aggregation of the leydig cells, aspermatogenesis, and increased excretion of urinary gonadotropin."
So that's Klinefelter's syndrome, and it wasn't until 1959 that Jacobs and Strong in Canada showed many men with this set of symptoms had another X sex chromosome. HOWEVER, now we know MUCH more, well I do anyway. See me about a man on Facebook, he can tell you all about his Klinefelter's syndrome and his 46XY karyotype!
So we can also see Klinefelter's syndrome, the symptoms of the disease seminiferous tubule dysgenesis, do not manifest, reveal themselves, until after the onset of puberty. Klinefelter's syndrome does not exist prior to puberty. And it's a damn shame those who run Wikipedia can't tell the actual truth, settling for commonly held beliefs of most doctors and most other people. I know you even link to Understanding Klinefelter Syndrome - a guide for XXY males and their families - which states quite openly on page 5 "I never refer to newborn babies as having Klinefelter's because they don't have a syndrome" Arthur Robinson MD - Author of the NICHD study of XXY males.
The karyotype is one thing, the possible symptoms of disease that can develop from it are variable. Everything is caused by the additional X genetic material, in their right time. Educational and learning difficulties in childhood are not caused by hypogonadism, but by the additional X material, that doesn't have to be and entire additional X. But what use is modern literature and greater understanding when those who think they're in the know refuse to take it on board?