If there's a moral issue here, it's about the experience we put the monkey mothers through to conduct this experiment since they cannot reasonably give consent for it and are at a sentience level where this is a reasonable concern.

From reading the article, given the value of this research, I think the concerns were adequately addressed - namely, the fetus was not brought to term, it was removed via C-section, the monkeys seem to have been adequately cared for emotionally, and it's a reasonable statement that simian miscarriage is a concept they would understand to some degree and thus not be overly distressed by the process. These were marmosets, not chimps so this is a reasonable level of precaution due their sapience level.