Even if siRNA works by the same mechanism in all vertebrate cells there are many steps were a difference between species can make it a failed treatment. For example there are many RNA viruses that can encapsulate replication machinery in certain types of cells, that helps avoiding innate immunity mechanism, if this happens in target cells in humans (may not be the same in monkeys) then the encapsulation would interrupt also the siRNA binding to viral RNA making it useless. also there is a posibility of interference of the siRNA against normal human mRNA important for cell life cycle hence toxicity and so on. Even the lipid nanoparticles could work less efficiently in the human body (sequestration by non target cells, lack of proper integration with cell membranes, activation of unusual immune responses not seen in 20 monkeys but maybe fatal in a fair percentage of humans, etc.)
Yes, it is most likely that the treatment can be useful according to their results until now, but there are literally hundreds of reasons why this could be not the case, it would not be the first nor the last time when a perfect treatment for monkeys proved to be useless in humans.