The call https://nl.wikipedia.org/w/api.php?action=query&list=exturlusage&eulimit=100&eunamespace=0&format=json&euquery=https://www.dbnl.org gives zero results, while the Dutch article https://nl.wikipedia.org/w/index.php?title=Theodoor%20van%20Ryswyck&action=edit clearly contains the URL "https://www.dbnl.org"
The equivalent call in the WPNL frontend seems to work: https://nl.wikipedia.org/wiki/Speciaal:VerwijzingenZoeken?target=https%3A%2F%2Fwww.dbnl.org
A modified call with a variant value of "euquery" does seem to work : https://nl.wikipedia.org/w/api.php?action=query&list=exturlusage&eulimit=100&eunamespace=0&format=json&euquery=dbnl.nl