Technically it uses Vulkan and is based on Google's Skia rendering library. Further it uses HarfBuzz for shaping and Winit for window management.
It is perhaps the single most impressive text editing visual design change I've ever seen (maybe second to kakoune's verb-noun reordering allowing selection-before-editing visualization). But while most such changes are functional but dull (eg sublime multiline editting), I can confidently say I like looking at this thing go (and the utility is minor, but obvious and good)...