The foundation had said it was taking Roets to court for defying an earlier decision by the North Gauteng High Court that banned the display of the old flag unless it was used for artistic, journalistic or academic expression.
In his papers, Roets denied that he defied the court decision as it was not against him or AfriForum.
He said the tweets were made in his personal capacity and not in his capacity as deputy chief executive of AfriForum.
He added that AfriForum was appealing the decision by the North Gauteng High Court.
The foundation had gone to court against the display of the old flag. The court found in its favour and banned its use.
But the battle is not over as Roets is now being taken to court for defying its decision.
Roets said in his court papers that he had done nothing wrong.
The foundation insists he was in contempt of the High Court judgment.
Political parties had supported the decision by the High Court.
They said the flag had no place in the new democratic dispensation and anyone who displayed it had no regard for the law and the constitution.
AfriForum is appealing.
The urgent application of the foundation against Roets is expected to be heard soon.