The prophecy of Jeremiah in chapter 25 doesn't mention the destruction of the temple in relation to determining the 70 year period.
25 The word that came to Jeremiah concerning all the people of Judah in the fourth year of Je·hoiʹa·kim+ son of Jo·siʹah, the king of Judah, which was the first year of King Neb·u·chad·nezʹzar*of Babylon. 2 This is what Jeremiah the prophet spoke concerning*all the people of Judah and all the inhabitants of Jerusalem:
8 “Therefore this is what Jehovah of armies says, ‘“Because you would not obey my words, 9 I am sending for all the families of the north,”+ declares Jehovah, “sending for King Neb·u·chad·nezʹzar*of Babylon, my servant,+ and I will bring them against this land+ and against its inhabitants and against all these surrounding nations.+ I will devote them to destruction and make them an object of horror and something to whistle at and a perpetual ruin. 10 I will put an end to the sound of exultation and the sound of rejoicing from them,+ the voice of the bridegroom and the voice of the bride,+ the sound of the hand mill and the light of the lamp. 11 And all this land will be reduced to ruins and will become an object of horror, and these nations will have to serve the king of Babylon for 70 years.”’+
12 “‘But when 70 years have been fulfilled,+ I will call to account*the king of Babylon and that nation for their error,’+ declares Jehovah, ‘and I will make the land of the Chal·deʹans a desolate wasteland for all time.
These verses clearly show that the prophecy wasnt specific to the Jews but all the nations around the area were to be subject to Babylon for 70 years.
It doesn't mention the temple.
It doesn't say the land will be in desolation for 70 years, it says that the nations will be in servitude to Babylon for 70 years.
Lastly, the 70 years were going to end, not with the return of the Jews to their homeland but with the punishment of the King of Babylon, which everyone agrees happened in 539 bce.
If the 70 years of servitude started when Jeremiah spoke the prophecy, that gives us a start date of 605, being the first year of Nebuchadnezzar. Is it possible that the 70 years had already started by then, as Babylon was already ruling anyway?
If the 70 years started at or about the time of Jeremiahs prophecy, that opens another can of worms for the WTS. Jeremiah 1 says that the first year of Nebuchadnezzar was the 4th year of Jehoiakim. According to jw.org, the 4th year of Jehoiakim was 624 bce.