I hacked it by replacing the storing of the ICommand in the Dictionary to keeping a reference to the original ToolbarItem instead.

readonly Dictionary<UIBarButtonItem, ToolbarItem> _buttonCommands = new Dictionary<UIBarButtonItem, ToolbarItem>();

In ViewWillAppear, replace the original adding to the dictionary line with:
_buttonCommands.Add(button,tool);

Then change this method to:

void ToolClicked(object sender, EventArgs args)
{
       var tool = sender as UIBarButtonItem;
       var command = _buttonCommands[tool].Command;
       command.Execute(_buttonCommands[tool].CommandParameter);
}

Seems to work ok.

FYI, thank you for this sample. It was invaluable for learning how to work with native iOS through Xamarin.Forms... especially for this iOS newbie.