I think this is likely to be the truth. It is difficult to make nations pay debts. If the EU want UK cash, this will be a negotiating issue. The UK could walk away, the EU then could undertake prolonged legal action at the ICJ to enforce the debt, the problem is any enforcement is done via the UN security council on which the UK is a permanent member with a veto. So effectively the EU will be unable to enforce any decision against the UK. Equally there will have to be negotiations over a lump sum payment vs. staged payments, currency, interest.
So since this is effectively an unenforceable debt to get payment the EU will have to offer incentives to the UK. The EU does itself no favours by bigging up the debt, as any smaller amount agreed at the end of the negotiations looks like a loss for the EU and gain for the UK.