I thought it was too but looks like the information is misleading. I just did a reservation quote on the excessbaggage https://www.left-baggage.co.uk/index/booking site it shows £31 for each bag and Ba wants £20 if I book it online each way makes it £9 more.

I called BA and when I asked her the same question she just kept repeating I linked your reservations so you should be ok but I can't guarantee anything.