That editor's note is by Mr. William S. Pelletreau, who must have been immersed in ephemeral information of New Netherland when he was compiling those volumes of wills. I assume he had a basis for that assertion, but unfortunately modern notions about source citations had not been adopted in his era. Maybe there is a court record in which Engeltje testified to her age.
As for the age at marriage: Do not assume that the standard rules of the Dutch church were efficiently enforced in New Amsterdam in 1639. This was a very new mercantile settlement at the frontier of European civilization, not an established city of Dutch burghers. Women of marriageable age seem to have been a scarce commodity, and there were other marriages of girls who were age 15 -- or younger.