Thing is, the insights on mustard gas in WW1 didn’t go much beyond: “Geez, this shit is nasty”, whereas in the aftermath of this WW2 one-off, one military surgeon noted that mustard gas appeared to interfere with the cell-splitting process, and his boss then realised that this could be made useful against cancer…

AFAIK the term “chemotherapy” wasn’t used before cancer chemotherapy, and it was adopted as an analogous construction to “radiotherapy”. Just as in cancer radiotherapy the radiation is targeted to destroy the cancerous cells, in cancer chemotherapy those cells are “chemically targeted” for destruction. The insight that cancerous cells could be more vulnerable than non-cancerous cells to some chemicals was gained in the aftermath of the Bari disaster, by the military surgeon reporting the effects of mustard gas on the sailors.

Not exactly therapeutic: it’s toxic, but as a WW2 military surgeon found out, it turns out to be even more toxic for cancer cells than for regular cells…

Also, it’d be just like Orwell to use this literary device in a tip of the hat to its appearance in the opening sentence of the most famous Spanish-language novel of all time.