It clearly states that. And also give an example. If you did not redeem them on IB+ you will have for sure a negative balance and with the IB+ and BAEC merging, this amount will be negative at the other side. But it is up to you.
My guess is that for 6 months the combine my avios link will have some 'technical' issues and it will not work at all...
It says you must use them with Iberia Plus, not Iberia - they are two
very different things...