If you changed 
const targetInterval = 700; // ms

to 

let targetInterval ; // ms

and then in the initializeMovingTargets function added the third line below, 


function initializeMovingTargets() {
    targets = targets_config.targets;
    targetSlots = targets_config.target_slots;
    targetInterval = targets_config.target_interval ;


it would actually use the target_interval value from the moving_targets_config.js 

Right now, the target interval is 700ms regardless of what they put in the config file