Except China's perception is beyond the buffer zone, which was a compromise. China initially asked India to vacate the entire Galwan valley, vacate Dhan SIngh Thapa, and stop building infrastructure in Chinese claimed territory. India refused all those demands, which is why there was a stalemate until India occupied heights on the southern bank. It was China that had to withdraw on the Pangong north bank and dismantle infrastructure. That is the exact opposite of a "fait accompli"
Going by your assertion, prior to India gaining advantage in South Pangong Tso, China held all advantage. Which is true because China was able to push back India from P8-P4-toP2/3 and gain advantage most of the areas north.
But then that would mean
no clear path for disengagement. It could lead to a hot war especially up North of Pangong Tso.
Do you think either China or India would've simply relented to the unilateral demands from either side? If anything, the least violent solution would be a lukewarm border always in an active state of confrontation - Much like India-Pakistan border.
Such a situation is not of interest to China.
China indeed demanded entire regions be evacuated fully cognizant of the realistic solutions it could expect.
And thus China kept discipline as down South, India inserted itself, with mechanized regiments, into the Kailash ranges relatively
unopposed. China didn't bother to oppose, almost as if China wanted India to gain a card so as to make the idea of disengagement possible (and digestible to the masses).
With the Pangong Tso agreement, China forced India to hand in the Kailash "pass" entirely ( that China "gave" it) and IA had to
retreat from Kailash ranges.
If India gained Kailash ranges over hard fighting with China, it wouldn't have traded it as readily. But it did. That's a question you might want to ask Indian leadership itself.
When viewed together with the aggression at Galwan, China's action can be seen to be focused on increasing pressure at areas North of Pangong Tso and laying groundworks for future buildup there. Because that's the region that holds the key to Siachen Glacier, Karakoram pass and Tibet - Xinjiang, China Pakistan economic Corridor.