That's mean they just buying time.....
They are looking to 'settle' the border issue with India.
Please recollect the original offer. Just for reference:
"Drop your claims on Aksai Chin and Tawang."
qz.com
In my personal opinion here (and I write my personal opinion when I am offering my opinion based on my appreciation of the situation), the Chinese read that Mr Modi has the necessary Political Clout and Capital to reach a settlement. Yet, they also understand that the situation may not be ideal as anyone compromising on the issue will also end up facing a public wrath. So, a 'carrot and stick' approach is being adopted, with a view to put pressure on GoI and the citizens as also create
The Chinese need India to reach some kind of settlement for u/m reasons:
1. To protect it's interests in world forums. A 'friendly' and 'compliant' India will be an useful ally in offsetting the debilitating fallout of the COVID 19 crisis. The same, the fallout, has hit the domestic economy severely. A potentially precarious condition for CPC's hold over power.
2. To prevent a formation of a formalized 'Quad' with potential allies in terms of nations facing security challenges with China joining at a latter date to augment the same into a NATO like structure. Please recollect that the primary reasons for the fall of USSR & Iron Curtain was simply the high economic costs. That includes everything - from technology R&D to market/finance access. The NATO & allied states benefited from lower developmental costs for Technology which fuelled the innovations for both the market and resulted in what was popularly known as "RMA"(or Revolution in Military Affairs). These again imposed costs on the USSR that it simply could not afford but had to.
(this is what we see happening with Pakistan, albeit in a modified way)
3. Securing CPEC. I have mentioned it elsewhere, Chinese will not hesitate to sell Pakistan down the drain if we were to insure and ensure their interests over CPEC. When the HM makes a statement over GB and in the same breath, Aksai Chin, their insecurities over this critical aspect - of finding an alternate to their precarious SLOC, come to fore.
4. India remains a very large market for them. Yes, it may seem counter productive to use force, but there is a rather typical Chinese logic behind it - the Indian core manufacturing is very selective and limited. Most of the 'Made in India' products that are of mass use, are manufactured by Chinese firms and Indian 'manufacturers' simply paste their company name and logo on it.
Of course, one must never forget that for the Chinese, the treaty is meant only to be valid for that time period. They can move out of it whenever they feel that it is invalid with 'change in circumstances/conditions/time'