So it turns out that I am the donut-hole here, I was using an outdated version of square.js that my angular schematic builds to my dist.
So the script that I was trying to get from the CDN was never working, or was being shadowed by what was already pre-bundled with the app.
With my local square.js script now the right version, my card form is now rendering properly:
I hope someone else finds this useful.