That the 70 years is a period of nations serving Babylon and not a period of exile is absolutely correct.
He burned down the house of the true God,+ tore down the wall of Jerusalem,+ burned all its fortified towers with fire, and destroyed everything of value.+ 20 He carried off captive to Babylon those who escaped the sword,+and they became servants to him+ and his sons until the kingdom* of Persia began to reign,+ 21 to fulfill Jehovah’s word spoken by Jeremiah,+ until the land had paid off its sabbaths.+ All the days it lay desolate it kept sabbath, to fulfill 70 years.+
It is very interesting that after Jerusalem was destroyed as mentioned in the verses above, the land was not desolate because some Jews remained under a governor named Gedaliah who was ironically assassinated by a man named Ismael. It was after this event that the king of Babylon sent the remaining Jews to Babylon and the land began to be actually desolate.
Obviously, the destruction of Jerusalem had to do with the conduct of the Jews and not the nations so the 70 years mentioned in the quoted verse applied to the Jews whether or not it also applied to other nations concurrently or with other starting and end dates.
Since WwI started in 1914 which is based on 607 gives JW confidence that 607 is the date of Jerusalem’s destruction, among other signs that JW also claim to have witnessed. The minds of JW believe which doesn’t prove they are right but there certainly is a reasonable basis for 607 as JW interpret the Bible.