Your presumption is correct. When a plan is overfunded, think of it as prepaying its future obligations, so the overfunding generates interest income rather than interest expense.
Under IFRS, Net Interest Expense/Income can be broken down into (PBO x r) - (FVPA x r). If FVPA is larger than the PBO, you will have Interest Income, which will reduce the pension expense reported on the P&L.