Whereas most Zionists are largely atheist but also believe that the land is theirs because a God(who they don't believe exists) gave it to them. But you consider those entirely rational
You really are out of your depths here.
Its what they have believed for thousands and thousands of years. Its how the Kingdoms of Israel were first formed. Its how they came to exist as a people.
When atheists Jews talk about that, they don't mean it literally. Its out of veneration of their ancestors.
You see, Judaism (unlike most religions) does not evangelize or seek converts. Traditional Judaic law does not even allow converts nor does it allow interfaith marriage.
Thus they have become a distinct ethnic group as a result of how they historically lived. Its part of who they are even if they don't believe in god.