As I had no replies on the thread, I thought I'd help out the community by testing it myself.
Went ahead and purchased the £409 GBP cable (here) to test on my 2021 QN95A, and despite my optimism, it did not work.
So there you have it folks, the 2020 cable does not work on the 2021 QN95A.
PS: I only tested this as I need a cable larger than 5 metres, but 5 metres is the largest option I can find for the 2021 models. But it doesn't make sense - looking at the cable connectors side by side, they look exact the same. At the same time, on the box, it says the cable supports up to 8K and 40GBps transfer rate, same as the 2021 cable.
Possibly a limitation/block software side.
The TV did make a strange noise when testing the 2020 cable, but the screen never turned on.
Hope this helps someone!