I just looked at your codepen, and did the following:
tl
.staggerFromTo("#main div", 0.5, {
transformOrigin: "left center",
autoAlpha: 0,
ease: Power2.easeInOut,
transformPerspective: -1000,
transformStyle: "preserve3d",
rotationY: -180,
}, {
transformOrigin: "left center",
autoAlpha: 1,
ease: Power2.easeInOut,
transformPerspective: -1000,
transformStyle: "preserve3d",
rotationY: 0,
}, 0.5)
So if you do not want (or cannot) change the CSS, just change the JS.
Another advice: to create the folding effect, change the rotationY (in the "from" part of the .staggerFromTo()) to -90 from -180, because the first div's back can be seen before it folds in.