Dutch law does not apply to other EU countries where "legitimacy" for civil law does not correspond to "legitimacy" for nobiliary law.
Benjamin Lascelles, eldest son of the British Earl of Harewood, is indeed legitimate for modern British law, but he remains legally incapable of inheriting the earldom because of his birth out of wedlock.
In Belgium, the standard remainder for nobiliary titles is inheritance in the legitimate male line only.
https://diplomatie.belgium.be/en/services/Protocol/nobility_and_honorary_distinctions/nobility/faq
Duc_et_Pair was accurate in naming the Dutch nobility, where there is no distinction for titles of nobility due to a legal amendment in 1994.
In the Dutch Royal House, the distinction is between children born from an approved marriage and children not born from an approved marriage.
Hugo Klynstra was his name at the time he applied to take his father's title and name.
The ducal title is a pretended one and is not legally recognized as a title of the Dutch nobility. If it were, then I suppose Hugo would be its heir indeed.
The Dutch Royal House continues to distinguish between children born in and out of approved marriages.
Delphine asked for the same rights as her siblings, but it is unclear whether the court granted her request. Only the titles and last name were explicitly confirmed by her lawyers.