The new 8KX and old 8KX require different firmware which is not compatible with each other because the chips being used, while performing the same function, are different and incompatible and require different code to run. This announcement concerns only the firmware for the new 8KX.
It’s possible that the same approach that reaches 120Hz on the new 8KX could be applied to the old hardware to improve compatibility for its 90Hz, but this seems unlikely. It’s probably utilizing capabilities that are only present in the new hardware. In any case, it would require an update to the firmware for the old 8KX to implement it if it was possible.
If you’re talking about trying to run the firmware for the new 8KX on the old 8KX, the headset will not be able to run at all. Don’t try this.
If you’re referring to trying the Lindy Repeater in the hope that this will cause 90Hz to work on an old 8KX which is having trouble with 90Hz, that is purely a flail that has been suggested on these forums recently. There’s no particular reason to think it will make a difference outside of the specific case (the 3090 Ti) where it has been recommended by Pimax as a workaround.
If using a repeater solved 90Hz issues on the old 8KX, that would almost certainly have already been discovered long before now. The Lindy repeater has been around for a couple of years.