This doesn't match the phrasing in the presentation, I would expect "the graph has diameter 20" to be defined without any reference to the weights of the edges

This doesn't match the phrasing in the presentation, I would expect "the graph has diameter 20" to be defined without any reference to the weights of the edges (whether in flying time, total time, cost...) But it's definitely plausible the author half remembered the routes with 20 hops from some optimized search like this, and wrote the wrong thing in the presentation.