Addendum to “Classification of marginally trapped Lorentzian flat surfaces in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:msubsup><mml:mi mathvariant="double-struck">E</mml:mi><mml:mn>2</mml:mn><mml:mn>4</mml:mn></mml:msubsup></mml:math> and its application to biharmonic surfaces”