Inquiry into WWI's possible impact on philosophy does not require one to say that a given philosophical stance would not have been formulated but for the War. Instead, the idea is that younger philosophers might have been disposed by the War to pay more attention to that view than to others, or to be much more critical of other, rival ideas, or to be more charitable to that position by not dwelling upon its most contentious presuppositions, and so on.
A rough analogy: if we ask how some stretch of DNA first was put together, the explanation will focus on some micro-process involving the relevant chemicals and their interactions. But if we ask how a gene came to preponderate in a species, we shall have to expand our focus to take in environmental interactions, including the forces of natural selection.
Similarly, we might account, say, for Moore's disaffection with idealism in terms of the reasons and arguments that he published. But if we ask, 'Of the various philosophical positions in the air in the 1920s and '30s, why did analytic philosophy catch on in the UK?', our answer may have to encompass social or political considerations in addition to the arguments that appeared in philosophy journals.
So, when someone advances a hypothesis about WWI's impact on British philosophers, it's no objection to say, 'Ah, but the philosophical ideas in question had already been formulated and defended in print years before the War, so they're not explained by the War.' This is because the question being posed is along these lines: of all the extant philosophical positions in the UK before the War, why did these ones catch on and preponderate among UK philosophers after the War?'