Both are using historic precedent to change current boundaries.
Are you claiming that the tibetans didn't sign the treaty with the British? The Chinese don't have to sign a treaty with the British. The tibetans who ruled those lands during the time did and china annexed Tibet. It is now their responsibility to honor those commitments made by Tibet.
Let me give you an example..
Say you purchase a company with massive debts. Can you now suddenly wipe those debts out because you didn't
take on those debts, it was the company you purchased that did? No!
When you acquire a company, you're taking on their assets as well as debts.
If the tibetans had acquired more land since this treaty was signed, would china reject those lands because they weren't part of the Chinese map from the days of yore?
Think about it!
Whether the Chinese case is stronger than the Afghans cannot be judged by you and I. What you're providing is opinion. Which is not what I'm asking for. I'm drawing a similarity betweeen the two situations.