China doesn't even recognize an LAC in Arunachal. It only recognizes an international border with Assam(it's claim)Because China made the claim on AP (Arunachal Pradesh) after India made a claim on AC? Therefore is seems to suggest China made the claim only to try and get India off its claim on AC.
Basically the thinking from China back then was that "okay India out of the blue is making a claim on our territory. How do we respond?" and the decision ended up being "we make sure they don't capture the land the Indians claim (done) and we make a claim on Indian land as well since we can't just let Indians make random claims without responding in kind".
Why else did AP only get claimed by China a while after India made the claim on AC? Why wouldn't it have been both at the same time? The fact is that the AP claim is associated and grew out of the AC dispute.
If India makes a claim on Tibet proper, you can bet your ass that China is going to make a claim on India proper as a response (after India claims let's say up to Hotan).
If China didn't then India can keep making random claims. The only way to prevent that from a strategic pov here in this specific case of Indo China dynamics is by making a claim as well so that India has something on the table to potentially lose as price of making random claims.
Would you make a bet where if the other person wins, they take half your stuff and if you win, you get nothing?
Do you understand the simple motivations and reason behind AP claim by China?
China also refused to exchange claims on AP/South Tibet for AC unless India recognizes Taeang and other border areas as Tibet.