Daniel 9:2 — 2 in the first year of his reign I, Daniel, discerned by the books the number of years mentioned in the word of Jehovah to Jeremiah the prophet to fulfill the desolation of Jerusalem, namely, 70 years.
Right "to fulfill" or complete. It's not a statement of equivalence. Daniel, in the first year of Cyrus, was reading Jerimiah and knew it was 70 years of Babylonian reign. That reign has just ended, and he knew that the 70 years was over, and the punishment could be completed / fulfilled.
Note: if you read this as saying "Jerusalem was destroyed and the Jews were in Exile for 70 years" then you set up an explicit internal contradiction in the Bible, since Jerimiah's words were crystal clear. You sure you want to do that?