Well I know the M1 Pro/Max can do that. But they don't do it with a single Thunderbolt port.
6K with DSC only requires HBR2 link rate. Two of those can fit in a single Thunderbolt connection.
If Apple is forcing dual HBR3 mode instead of single HBR2 with DSC, then it means there's not enough bandwidth to write to the XDR's USB at more than 1 Gbps.
It don't look like they've replaced a TB port, it looks like they've added one to the TS4. DisplayPort was also on the TS3+ (and TS3 lite for that matter).
Sure, compared to a Thunderbolt 3 dock which has only 1 downstream Thunderbolt port, the TS4 with two downstream Thunderbolt ports is an upgrade. But a Thunderbolt 4 dock can have 3 downstream Thunderbolt ports.
Looks like a great hub. 2.5 GbE is a little strange. I suspect this is due to bandwidth constraints. mGIG switches are still unreasonably expensive for some reason (compared to straight 10 Gbit hubs). 1 GbE would be cheaper and mGIG users can easily find USB-C and Thunderbolt based solutions.
What bandwidth constraint? USB goes up to 10 Gbps.
What I've read about TB3 PCs is they work with TB4 docks but they can't take advantage of the TB hub ports. So you get 1 downstream port in earlier TB3, this one has 2 downstream ports that TB4 hosts or TB3 Macs can use, so basically the other ports work but you can't plug 2 TB devices in at once, only one. Apparently Macs with TB3 can use TB hub though, so it's a bit confusing.
I think this is because macOS uses a Thunderbolt software connection manager. PC's rely on the internal connection manager (ICM) in the firmware of their Thunderbolt controllers.
Linux has added support for software connection manager. Maybe it allows Thunderbolt 3 PC's to use Thunderbolt 4 docks?
One difference between the Alpine Ridge based TS3+ dock and Thunderbolt 4 docks is that the TS3+ has four USB controllers and a PCIe Ethernet controller so it can use all the bandwidth of Thunderbolt (≈22 Gbps) by itself (but two of the USB controllers are 4 Gbps, one is 8 Gbps, and only the Thunderbolt USB controller is 10 Gbps).
Thunderbolt 4 docks are usually just USB hubs so it can only use 10 Gbps by itself (but at least all the USB ports support 10 Gbps). The Goshen Ridge Thunderbolt controller has the option of connecting 1 PCIe device (8 Gbps) but I haven't seen it get used before. You have to connect non-USB things (displays or Thunderbolt devices) to use more than 10Gbps of bandwidth.
Maybe the TS4 uses a PCIe 2.5 GbE Ethernet controller but probably it's just a USB adapter.