Remember: unpkg.com/topojson@2 is not d3js.org/topojson.v2.js
unpkg.com/topojson@2 is topojson/topojson and converts GeoJSON to topologies.
d3js.org/topojson.v2.js is topojson/topojson-client and knows what to do with topologies.
Built with blockbuilder.org
https://d3js.org/d3.v4.min.js
https://d3js.org/d3-geo-projection.v1.min.js
https://d3js.org/topojson.v2.min.js
https://unpkg.com/topojson