Earlier quoted context omitted.
More precisely: the first letter of a name is always capitalised. So in "Hans de Zwart", the 'H' is the first letter, and therefore capitalised. But in "De Zwart", the 'D' is the first letter, and therefore capitalised, even if it normally wouldn't be. There might be an exception to that if that letter is not part of a whole word. "De" is a word, but I once knew a guy whose last name was 't Zet . The "'t" is not a wo…
Since we're bringing up fun facts about Dutch capitalisation, when the first two letters are "IJ", which pronounced as a single letter (a vowel) in Dutch, then both are capitalised.
(Personally I think it's two letters, but there are very serious sources, including a major encyclopedia as well as primary schools, that disagree. In games and puzzles it's also usually considered to be a single letter.)