just to be clearer: Line Bender organization was started by Raph Levien, and he informally leads and drives the work forward.

And he is the first author of the paper (and he is also commenting in this same thread)

I believe metafont, created by Knuth for Tex, is an exception here in that it does use strokes.

The technique works for both fills and strokes, and is used by Vello for both, including for the text rendering. One of the nice things we found is that lowering Euler spirals to arcs works particularly well, and you need many fewer arc segments than line segments to accurately approximate a smooth curve. We haven't plumbed arcs all the way through the rendering pipeline yet, but it's planned; the math for rendering arcs is not that much more complicated than lines.