Sorry, I just added this, but it came too late for your reply:DSC on Mac for third-party displays is a bit of a contentious issue haha Apple actually broke it with Big Sur for third-party panels. I think some still have it working but it's a big thing. We've had alt-mode on Thunderbolt and the ability to send two thunderbolt signals through one cable for a long time which is separate to DSC but it's part of how the 6K display works.
You seem to be saying older Macs (without DSC), when running the XDR, will be running it at lower resolution. However, the following post says that they do maintain 6k res even without DSC:
A 5700 iMac uses a single DisplayPort 1.4 HBR2 with DSC signal (6016x3384) to drive an XDR display so it can drive two of them no problem.
Other Macs that support DisplayPort 1.4 but not DSC use dual HBR3 (each 3008x3384) to drive an XDR display at 6K.