The title is very misleading. Deno is not written in JavaScript and is never referred to as Deno.js in any official sources. Deno is written in Rust.
Much of Node.js is written in C, yet it's still called Node.js.
Deno has some JavaScript/TypeScript in it. On GitHub https://github.com/denoland/deno is 22.8% JavaScript and 13.2% TypeScript, and https://github.com/denoland/deno_std is 68.2% JavaScript and 31.6% TypeScript.
So to me the title is misleading about the name (Deno is certainly not named Deno.js), but not about what Deno is written in.