The message relates to hidden city ticketing, and doesn't apply here at all. It's a standard condition. You'll find it on pretty much all tickets I think.

The idea is that if you were to buy, say, a DUB->LHR->HND return, and decided to short check your back on the return so you actually flew DUB->LHR->HND->LHR (and stopped in London), the ticket might be cheaper than booking that itinerary. So in theory BA could charge you the fare difference between a straight return back to DUB and an open jaw terminating in Heathrow. There are endless schools of thought about whether this would be enforced, and whether it's the slippery slope to damnation, but that's the reason for the condition.