Isn't King Willem-Alexander also Count of Buren?
Yes and no. When the so-called "Bataafsche Republiek" was vested in 1795, all lordships, baronies, viscounties, counties, marquessates, duchies, principalities, but also all ecclesiastical territories, cities' rights, etc. were declared null and void to establish a central and unitary government.
The Orange-Nassaus however continued the use of their forty-something titles in the so-called Intitulé (the Etc. Etc. Etc. in "Willem-Alexander, by the grace of God, King of the Netherlands, Prince of Orange-Nassau, Etc. Etc. Etc."). Their argument was that the Bataafsche Republiek had no jurisdiction on titles which were not their Fons Honorum. For an example: the title Count of Buren was created by the Holy Roman Empire in 1492.
In 2002, during the reading of the modernized Royal House Act, the government stressed that these titles became not extinct in 1962 when Queen Wilhelmina, the last male-line Orange-Nassau, passed away. The Government argued that these titles are connected with the kingship and that therefore it passed from Wilhelmina on Juliana on Beatrix.
So yes: the titles are still there. And no: the titles are not used anymore in real life.
Duke of Limburg
Created 1839
Fons Honorum: by Treaty of London
In the Royal House since 1839 (by creation)
Marquess of Veere and of Flushing
Created 1551
Fons Honorum: the Holy Roman Emperor
In the Royal House since 1582 (by abjuration of Philips II of Spain)
Count of Katzenelnbogen
Created 1066
Fons Honorum: the Archbishop of Cologne
In the Royal House since 1500 (by testament)
Count of Vianden
Created 1090
Fons Honorum: the Abbot of the Benedictine Abbey of Prüm
In the Royal House since 1417 (by testament)
Count of Diez
Created 1073
Fons Honorum: the Abbot of the Stift Sankt Viktor in Mainz
In the Royal House since 1388 (by testament)
Count of Spiegelberg
Created 1200
Fons Honorum: the Holy Roman Emperor
In the Royal House since 1631 (by testament)
Count van Buren and of Leerdam
Created 1492
Fons Honorum: the Holy Roman Emperor
In the Royal House since 1558 (by testament)
Count of Culemborg
Created 1500
Fons Honorum: the Holy Roman Emperor
In the Royal House since 1748 (by donation)
Viscount of Antwerp
Created 1106
Fons Honorum: the Holy Roman Emperor
In the Royal House since 1521 (by testament)
Etc Etc Etc