That's a red herring: video on Apple Silicon is implemented internally using DisplayPort and Thunderbolt and the HDMI output on Macs is driven by a DisplayPort-to-HDMI converter (
https://www.ifixit.com/News/54122/macbook-pro-2021-teardown). It's not going to be driving anything via HDMI that DisplayPort 1.4 can't drive.
Just to be clear, I'm not claiming that a 5K120 display is anything more than a possibility: just that it's perfectly feasible for Apple to implement DP 2.0 staring on the next generation of Macs and that there's no reason Apple would worry about a brand new display fully supporting older Macs (which could run it at 5k@60Hz). Maybe they will, maybe they won't - but that's different from claiming that it is
impossible. Nobody here knows (including the analysts, as they've shown with the last couple of rounds).
A new 5k display at WWDC - only 3 months after the launch of the Studio Display - was never credible.
"Many expected" the new Mac Pro (with previously unseen Apple Silicon) at WWDC, as well, alongside the "real"
3nm M2, M2 Pro, M2 Max and the new MacBooks that use them... all of which
could have supported a new display. All we know about
any of those rumours is that they
all turned out to be wrong, so you can't tell anything one way or the other.
The relevant "timeline" here is the
DisplayPort 2.0 standard, which was formally released in 2019 and devices were originally expected to appear in 2021. There was plenty of time for Apple to
plan to incorporate the tech in 2022/2023 devices. Of course, DP2.0 got delayed just like Apples launches probably did. Again - I'm just saying that DP2.0 in 2022 was
feasible - not a certainty.
Perhaps at a price point somewhere between the 7k XDR (probably about $5000 going by the 6k version) and the Studio Display (which still ain't cheap). Or maybe replacing the Studio Display (which could be a year old by the time the Mac Pro finally appears).
They're not developing it "at the same time" - the Studio Display has been out since March and I wouldn't expect
any new display - whether it is 120Hz or just miniLED/HDR at 60Hz (which many people would be happy with) to appear until
at least 6 months, possibly 12, after the existing Studio Display and the Mac Studio. Typical dates for Mac launches would be October/November or next March. Apple are
not going to lose sleep if a (by then) 1-year old computer can't drive the new display at 120Hz. It could still drive it at 60Hz and full resolution.
On the other hand, the Studio Display got a lot of flak because the screen technology was only slightly better than the 2017 iMac, while the MacBook Pro etc. had got fancy "XDR" branded miniLEDs - so it could stand an early update even if it was still 60Hz - and a miniLED version would probably re-use much of the same design.
If the Studio Display was "just for the Mac Studio" why would they give it a massive, expensive slimline power supply that could charge at 96W - completely wasted on the Mac Studio?