"The original ECD program, which lasted two years, enrolled 127 stunted children aged 9-24 months. Researchers randomly assigned these children to one of four groups: a psychosocial stimulation intervention, a nutrition intervention, both interventions, or neither intervention (the comparison group)."

That was my first thought too: the positive results could be effect of special attention and expectations (placebo effect). However comparing "both interventions" group with each single intervention group should address this.