Not sure I buy that. Seems like avoiding the double negative is worthwhile. Not all default values need to be false, do they?
Agreed. It's a bad UX convention even; such as error alerts reading
"Are you sure you want to not save the document? [YES] [NO]"
If you consider that not setting it results in undefined which would evaluate to false then they are correct. But I don't think that needs to be the case. In reality you can default to whatever you want in which case I agree with you.