I don't know why the sibling comment is
dead (edit: was). I mean, it is a valid concern, if one doesn't know.
So, for slickytail and anyone who has the same question:
The code is actually compressed into a binary blob. You can see it if you just look at the source of https://0b5vr.com/0mix/0mix.html
A small script loads the blob and uncompresses it before running it through eval:
fetch("#").then(t=>t.blob()).then(t=>new Response(t.slice(156).stream().pipeThrough(new DecompressionStream("deflate"))).text()).then(eval)
This is a common approach in browser demos and what is counted as "less than 64Kb" is that final html. A similar technique compresses it into a PNG.