Your premise is wrong. A sanctuary city is by definition a city and not a border. So yeah, "open borders" is a strawman.
You're trolling. You're trying to be smart by playing semantics, taking the literal definition of border as a physical line.
Borders are an abstract concept. The border of a country is not the lines on a map. Look at airports and embassies, for example. If you're at an airport in Toronto and step into the US customs area, you have crossed the border to the US legally, without having physically crossed the demarcation line.
A country can be considered to have open borders by definition if the law associated with unauthorized entry isn't enforced (de jure). So he's right that if you have areas where people can run and avoid prosecution for immigration law violations, you have a de jure open border. It's common sense that your border doesn't mean shyt if there's no penalty for violating it.
An open border is a border that enables free movement of people between different jurisdictions with few or no restrictions on movement, that is to say lacking substantive border control. A border may be an open border due to a lack of legal controls or intentional legislation allowing free movement of people across the border (de jure), or a border may be an open border due to lack of adequate enforcement or adequate supervision of the border (de facto).