I've read here people expressing confusion why Apple is now advertising billion colors on new iMac when the previous ones some say did that already with previous system update, I right now stumbled on this apple website where it's now distinguished between them stating clearly that displays from 2014 and 2015 support millions of colors and 2017 displays support one billion colors, so that seems to make that this isn't software but actually new hardware. It looks like also these are first ever apple displays that support billion colors even though they're still not true 10 bit but dithering. Sorry for my poor English.
https://support.apple.com/en-gb/HT202471
https://support.apple.com/en-gb/HT202471