I imagine this as an Emacs package that recognizes tree-like code patterns (like what he showed in the video) and replaces those patterns inline when you toggle it. Your code is still the same plaintext. But your editor can display chunks of it as graphs.
Emacs does this already in latex-mode and org-mode for math formulas.[1] It shouldn't be a stretch to do this for other types of code, too.
[1] See this example in markdown-mode: https://external-preview.redd.it/ETo2U5C7vh5o4482sMcnYbkZ6Lx...