When we say "She has cancer" we mean "a cancer". (Hopefully she doesn't have all of them!). Why couldn't the sentence "AI will cure cancer" mean "a cancer" as well?
"She has cancer" is a specific event, so we logically deduce that it is "a type of cancer".
When you say "AI will cure cancer" that is not a singular event, so you assume cancer in the plural. You would have to say "AI will cure some forms of cancer" if you didn't mean the plural here.
When you say "AI will cure cancer" that is not a singular event, so you assume cancer in the plural. You would have to say "AI will cure some forms of cancer" if you didn't mean the plural here.