Actually, it is a mandatory feature of HDMI 2.1. The point is that AMD has been supporting it before it was mandatory.
en.wikipedia.org
You don't seem to understand that FreeSync and VESA Adaptive Sync are the same thing. FreeSync is AMD's branding of their VESA Adaptive Sync implementation on their GPUs. And considering that the consoles will most likely use HDMI rather than DP, and use AMD hardware, it is more accurately labeled as FreeSync than anything else.
Ok. I found this article, that states exactly that;
"With HDMI 2.1, VRR support becomes a mandatory feature of the standard, forcing both TV makers and device makers to adopt support for variable refresh rates. Let's be clear, this is why Nvidia is supporting HDMI VRR. They have to support it to fully support HDMI 2.1. "
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Then again... Is that a positive thing? In this case, nVidia is only giving this to you because they are forced to. Is that a better picture than AMD that has been supporting it before it was forced?
And from that same article;
"AMD has already promised to add HDMI 2.1 VRR support to its Radeon Software drivers. At the time of writing Radeon FreeSync and FreeSync 2 support is already available on a number of Samsung TVs. "
And from your own Anandtech article and your own quote, you didn't highlight the most important point;
"As a result we’ve known for some time now that NVIDIA could support VESA Adaptive Sync if they wanted to, however until now they haven’t done this. "
In other words, they were holding out on their users because they wanted to sell their own stuff. Do you really think VRR would have become a mandatory feature if AMD did not push FreeSync? Honest question.