"United States Census, 1850," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MHVC-BBW : accessed 25 Jan 2014), John B Hedding, German, Vanderburgh, Indiana, United States; citing dwelling 132, family 132, NARA microfilm publication M432, roll 176.
The passengers on this ship were mainly German and they were the only Dutch on board, They might even speak the same language, because Groenlo is just a few miles from the German Border. So it definitely makes sense that people thought they were German too.
In the 1850 census it appears that most of the neighbours were of German descend, which makes the confusion even bigger