Conference was never valid, legal

After the 1911 Revolution, the 13th Dalai Lama called a meeting to discuss Tibet's future. Many people, especially those from lower social strata, opposed severing ties with the central government.

So the 13th Dalai Lama was hesitant to cut ties with the central government, especially as it amassed its army in neighboring provinces.

And the British government didn't want to drop "Tibet independence", so it convened the Simla Conference of 1913-14.