HN user
It's $ms-ratio: $golden;
When you bring Modular Scale into your project it will include a number of global variables including $golden with commonly used ratios in design. They're useful to try out when you're experimenting with different $ms-ratios. Check out the list here https://github.com/Team-Sass/modular-scale#ratios
The idea of using calc and VW units to linearly increase the font size is pretty cool. On real sites, though, it's important to keep control over exactly when the increases in scale are triggered. I'll set up some breakpoints just for the root font-size shifts, and have other breakpoints coordinated with additional layout changes I make within a design. The scaling and breakpoint settings tend to be unique between designs and non-linear.
I'm going to follow up to this post soon with a SASS mixin that gets around the @media query code bloat and lets you coordinate with globally named breakpoints.