that text doesn't appear on that page. and that's a very generalised statement covering a long period of time and wide area.
My understanding was that adults were rarely castrated, rather boys from certain regions were targeted (I believe boys from modern Sudan were preferred by the Ottomans)
"Castration: A fair proportion of male slaves were imported as eunuchs... in practice, emasculation was frequent"
(and yes, it did also carry a high rate of death.)