Reason:

Wikipedia based on academic sources https://en.wikipedia.org/wiki/Wikipedia:Attribution + https://en.wikipedia.org/wiki/Wikipedia:No_original_research, and not modern Photoshopped fan art flags, like this fantasy "Nitra flag", Wikipedia is not "Game of Thrones". What is the real academic source or evidence that a mentioned state used this flag?

The image description cleary states that is a fantasy work: "The origin of the flag comes from the 13th century, as a part of the Nitra's seal and also from the coin of the Gejza I. who was the prince of the Nitra Principality. The origin of this coin goes back to the times around the year 970. There is a possibility that this may be the flag of this principality."

Principality of Nitra was in the 800s for a short period, how possible to make a 13th century flag for an older entity? By time machine?

Exactly which flag from 13th century? Exactly which coins of Géza I of Hungary from 11th century go back to 970? Hungary used that symbol since Bela III of Hungary (who was the heir of Byzantine throne) since 12th century and not before: https://en.wikipedia.org/wiki/Coat_of_arms_of_Hungary

Slovak academic source: https://matica.sk/byzantske-korene-statneho-znaku-sr/ "No archaeological finds have confirmed the presence of a double cross in Central Europe during the Great Moravian period or the immediately following period. The oldest double cross in present-day Slovakia dates back to the 11th century. The exception of depictions on Byzantine coins, the double cross did not appear anywhere in mural iconography, in plastic form, or as a solitary object before the middle of the 10th century. It is a deliberate misunderstanding to make double cross from 2 single crosses on the Hungarian coins from 11th century. The double cross, a symbol of royal power, appeared only during the reign of King Béla III of Hungary (1172–1196)."