Quick question: I assume that doesn't mean "one divided by zero", but I'm confused what it is supposed to mean instead

That's a good question; I don't have a good answer. In honesty I copied the text from here (https://en.wikipedia.org/wiki/Stern%E2%80%93Brocot_tree) without thinking too hard about the meaning, simply wanting to implement the algorithm as the starting point for a tiny package that could reduce longer decimals into more human-friendly fractions. While I believe it does refer to the "undefined" fraction 1/0, from the algorithm's perspective of calculating the mediant and comparing it, it's somewhat negligible.