I've long been aware of that the claim that he carried out such an atrocity is nonsense, but had vaguely assumed that he probably did urge something like that- after all poison gas wasn't as taboo then as it is now and Churchill did come up with a lot of miserable schemes. However it turns out that even that version wildly distorts what Churchill actually proposed. Paul Bogdanor's list of 200 Chomsky lies (Chomsky is another who has spread this myth) provides the actual context for the phrase:
“It is sheer affectation to lacerate a man with the poisonous fragment of a bursting shell and to boggle at making his eyes water by means of lachrymatory gas. I am strongly in favour of using poisoned gas against uncivilised tribes. The moral effect should be so good that the loss of life should be reduced to a minimum. It is not necessary to use only the most deadly gasses: gasses can be used which cause great inconvenience and would spread a lively terror and yet would leave no serious permanent effects on most of those affected.”In other words Churchill was urging the use of tear gas as a means of minimising fatalities that might be caused with conventional weapons. Tear gas would not routinely be described as "poison gas" nowadays so using the phrase without any context to make it clear that he wasn't referring to stuff like mustard gas or Sarin is deliberately misleading.