Du Plessis said in an interview that Walter Smiles, who this week told the Truth Commission he threw the grenade at the Bophuthatswana consulate in Kimberley in May 1993, had made a similar confession to legal authorities shortly after the attack.
Smiles said the bombing attempt went horribly awry when the grenade exploded in the middle of a group of protesters outside the building, killing one person and injuring 39 others.
Du Plessis said: "We considered that statement in the light of all the evidence... at our disposal and decided that it was a pack of lies."
Two other men convicted of being responsible for the blast are serving 12-year jail terms. The commission recommended the men be released.
Smiles said he attacked the consulate during a protest march by the ANC Youth League on the orders of regional Umkhonto we Sizwe commander Lawrence Mbatha.
The commission also heard how police allegedly fabricated witness statements and suppressed evidence to ensure the conviction of suspects Sipho Mbaqa and Nkosinathi Nkohla for the blast.
Although Smiles came forward during their trial to make a confession, police allegedly turned him away, saying he would only complicate matters if he made a statement.
Smiles told the commission he intended applying for amnesty from the commission and asked that Mbaqa and Nkohla be released from prison as they were both innocent.
Smiles' confession caused some confusion among reporters who had copies of a written statement he had made to commission officials earlier.
In the statement he denied all knowledge of the grenade attack or any personal involvement.
Another witness to contradict his written statement was Thembinkosi Ngqele. In his statement he said he had seen Mbaqa hand a grenade to Nkohla during the march and had noticed the latter's hand moving "as if he was throwing a cricket ball".
Ngqele provided much of the State's evidence against the two during their trial.
When he took the stand on Tuesday, however, he claimed police had tortured him into making a false statement at the trial.