Evidence comes from the Jenova∙SYNTHESIS fight, where
Summoning Knights of the Round during this battle will give the main body of Bizarro∙Sephiroth an extra 60,000 HP and Safer∙Sephiroth an extra 80,000 HP.
This gives fans reason to believe there’s a connection between the two, and that the logical reason is that these knights were warrior Cetra who fought the invading alien. They were the small band of Cetra who “mounted a final assault on the being they referred to as “the Calamity from the Sky,” sealing its body at the alien’s point of arrival, the North Crater”.
Beside being unclear why this points to the Knights of the Round are Cetra, or that they even defeated Jenova 1000 years ago (as Jenova actually arrived 2,000 years prior), Kitase provided a cutting rebuttal in this article to speculators:
“Everyone’s thinking too deeply, reading between the lines too much,” laughed Kitase. “That makes it difficult because if you think about it that way, we might have to make it that way. That’s definitely not true.”
He also mentioned that Nomura created each summons in Final Fantasy VII, and none of them “have any background story attached to them”. But Kitase seems to hint that if fans demand it, they may change the remake to “make it that way”.