I solved this programmatically by adapting
Tanguy-G's answer for native Objective-C by setting AutomaticallyAdjustsScrollViewInsets to false in the constructor of my view controller.
public MyLovelyViewController (IntPtr handle) : base (handle)
{
this.AutomaticallyAdjustsScrollViewInsets = false;
}
EDIT: If you are setting any other aspects of the text view in question programmatically, make sure to call
this.AutomaticallyAdjustsScrollViewInsets = false;
after any other adjustments, otherwise the space will reappear, e.g:
confirmSummary.Editable = false;
confirmSummary.Selectable = false;
this.AutomaticallyAdjustsScrollViewInsets = false;
(I did this in the override of ViewDidLoad)