The short answer is prescription does apply. It must work if your pleadings are correct. You say you received a summons a few days ago. I’m not sure if you received a combined summons or the most likely one given the description of the debt a simple summons aka summons claim in respect of debt or liquidated demand. It calls on you only to enter your notice of intention to defend within a certain time which you have done. The Plaintiff must then file what is called a declaration(the very same things as a particulars of claim).

The further specific thing you mention is the unlawful clause. The short answer is that one of the requirements of any valid contract is that it’s very purpose, objectives and conclusion must be unlawful. It also can’t go against public policy or the ethos of other specific legislation created to cater for certain debts[National credit Act 34 of 2005(NCA)]. It also cannot mandate what a specific piece of legislation should be subject to. The Prescription Act 68 of 1969 is not subject to their clause. It speaks with legislative authority. Your case is further strengthened by the fact that the debt is prescribed, a new section has been inserted into the National Credit Act(among many others) to cater specifically for the prescription of debts and all matters incidental to it to such an extent that it says that no person can collect a prescribed debt or sell a prescribed debt.
Hope this helps