I think the point of the quote is that it's yet better to spend that time doing productive things unrelated to revenge, given that generating enough such contingencies is pretty costly.

No, it isn't. That is another point that could be made in the general area of "Boo Revenge". The most useful point that is conveyed, via assuming it as a premise, is that taking revenge is dangerous.

I argue that quotes don't (or rather shouldn't) get credit for all possible supporting arguments for the general position they are applauding.