"One display with up to 8K resolution at 60Hz or 4K resolution at 240Hz over HDMI"
The previous Macs supported so many 5k/8k display at once, they arguably should be able to drive a single 8k display, technically.
Example, Mac Studio supports up to 4 x 6k displays AND 1 4k at once. So that is 4 x 18M pixels + 8M pixels = 80M pixels
8k displays are 32M pixels, so Mac Studio is arguably already able to push 2x that many pixels...
Or maybe it's related to the HDMI chipset they chose specifically until now.
Now they support HDMI 2.1, based on this announcement, so they're finally catching up to everyone else. It would have been dumb for them to fall further and further behind on a display connector regardless of what monitors Apple has in the pipeline.