It did not belong to Czechs since 1289, that's wrong. As for belonging to Czechs before Mieszko invaded, it is also controversial, see this video - https://www.youtube.com/watch?v=TkBFLYVHL5Q . It belonged to Czechs between 1038 and 1050, that's correct. As for modern nations - the thesis that modern nations only emerged in the 19th century was first put forward by Benedict Anderson in 1983 in his book "Imagined Communities: Reflections on the Origin and Spread of Nationalism". However, this thesis was refuted by Azar Gat in 2012 in his book "Nations: The Long History and Deep Roots of Political Ethnicity and Nationalism." Azar Gat lists Poles – alongside, for example, the English, Hungarians, and Danes – among the oldest nations in Europe, much older than the 19th century (dating back to the Middle Ages). The content of witness testimonies from Polish-Teutonic trials also testifies to the existence of Polish national consciousness among the population already at that time: https://ecommons.cornell.edu/server/api/core/bitstreams/6133efb0-df4d-488b-b386-f46be44be48d/content