According to Hugo Grotius, it is not immoral to lie to someone who does not possess "the right to exercise liberty of judgment". He then proceeded to give two examples: (i) you cannot lie to a madman (ii) you cannot lie to a child. The argument is that lying is only immoral when it constitutes "Violation of Real Right" and these entities do not possess such a right because of their limited capacity.
So, is it correct for me to infer that it is less immoral to lie to those who are less intelligent, because they are less capable of or less likely to resort to this "exercise of judgement"? An analogous argument to Grotius' would be that it is less immoral to lie to a 3 year old than a 6 year old because the former does not possess much ability to comprehend. But age, as opposed to intelligence, is not a factor here. So we are essentially stating that it is more immoral to lie to a person with higher IQ (generalizing the 6 year old) than a person with lower IQ (generalizing the 3 year old).
The practical implication is that, if I were to run a scam, it would be less immoral for me to prey on more vulnerable groups, e.g., older people, or less educated people, compared to scamming people who are less vulnerable, say, other scammers (simply because they are well versed in the trade of scamming and possess a stronger ability to judge a situation of scam). This does not match my intuition very well.
So, is this "moral deceptionism" theory by Grotius a fallacy or is it that I do not understand him correctly?