The judgement is not made in the view that BA is absolutely expected to have replacement pilots at outstations, however that if any delays arise due to a pilot sickness, then under the definitions and purviews of EU/UK261, the consumer is entitled to compensation since, as per the judgement, this is under the airline's control (you could argue that, yes, BA should be expected to have stand-by pilots because this is completely under their control, unlike ATC decisions for example). Note that you may opine to the contrary (and I do agree it is, for av-geeks or people who know a bit more about aviation, this is not reasonable), but this is based on EU/UK261 definitions, which as you have probably understood from this judgement, are broad and designed to advantage the consumer. This is obviously touching the point of morality vs justice, a philosophical theme dating back from the ancient Greeks and still discussed to this day!