Apple has managed to shrink the thickness of the display assembly every generation so it wouldn’t surprised me they managed to do it again!
As to thickness of the current 27” 5K display its about 10 mm at its thickest point where the T-CON board is located. Review Step 5 & 6 here: iMac Intel 27" Retina 5K Display Teardown
Given the fact the 24” M1 system is 11.5 mm they clearly thinned it down a bit more. Even still there is nothing really on the backside of the display assembly other than a set of air chambers, WiFi & BT antennas, small I/O board and ribbon cables to get around all of which are thin. Here’s an educated guess on what’s inside!