It's just that "open borders" has been thrown around a lot. And to the unindoctrinated, it could mean...
1. Any one can enter the country without being stopped, asked questions or required to do anything;
2. There are no barriers to entry to drugs or, shit, goods from any country.
3. Any foreign country can move troops here and establish a military base.
But no, it means something less, like giving healthcare to people who sneaked over the, well, open border that apparently you didn't need to sneak over.
It's like the "war on women". It could me that one may kill women at will, because we are at war with them. Or it could mean that you have a different opinion as to whether mothers should be allowed to murder their unborn children. It's a bullshit incendiary term that trivializes a complex topic.