Nation-state and state are two different concepts, don't meddle the two.
"State" refers to a single political entity.
"Nation-state" refers to "state" with a homogeneous people and culture. (Post British era, India had become a state, but not a nation-state. Never will be, due to its diverse ethnic and cultural background)
You need not be a nation-state to be a state/a single political entity. As with the case of most ancient kingdom/empire. China has been a single political entity since 221 BC, adding or subtract Xinjiang/Tibet makes no different. If USA losing Texas or California back to Mexico, it doesn't alter the status of USA as a political entity as it ha been created in 1776. If India add Sri Lanka or loses kashmir, it doesn't changes the status of India today either.
If every ethnic or cultural group is considered as a separate nation in itself then most nations of the World would not qualify to be nation states rather be multinational states. This includes Pakistan, India and China too as all of these are ethnically and linguistically diverse countries. While State is a political entity, nation-state adds cultural identity to the state. Indian nation state concept is derived from common culture with different ancillary sub-cultures. More than 80% of Indians are Hindus and they are spread from north to south. Nation state concept of India may not be same as that in Europe.
India was also ruled by kingdoms like China. Almost half of modern day China was not ruled by central kingdoms for a large part of history. Same way central authority based in Delhi did not rule most of India for a large part of history.
Below is the Imperial Edict of the Abdication of the Qing Dynasty Emperor. The content circled in red is: Lands from Manchu, Han, Mongolia, Uyghur and Tibet continues to form an integrated China.
View attachment 375734
Qing dyansty was not a nation. During the revolt of 1857, people reinstated last Mughal ruler Bahadur Shah Zafar to the throne at Delhi and proclaimed him Emperor of India. Most princes and kings accepted him as their emperor.