Solved it by bypassing the problematic syntax and using square brackets, like this:
let timelineInit = wixAnimations.timeline({"repeat": -1});
const myImageInit = $w("#vectorImage118");
const myImageInit2 = $w("#vectorImage119");
timelineInit
.add( [myImageInit,myImageInit2], { <<==============
"rotate": 360 * 100,
"duration": 20000 * 100,
"easing": 'easeOutQuint'
} )
But the syntax in the API reference most probably should be corrected, in this specific example. It’s the Code Example snippet entitled “Add animations on multiple page elements together”.