The appeal of "Pure JavaScript" that they're alluding to is the that this library can be used in any compliant JS environment. The value proposition is no platform dependencies.
The description says "Pure Javascript manually written". But it's not. It's actually TypeScript, which if you set up the right environment and install Microsoft tools - it can be compiled to JavaScript. This makes new developers believe that JavaScript is a Microsoft product and that you need Microsoft tools to develop it.
* the distributed bundle is always javascript,
* it is manual re-write from BLAS reference.
* Typescript is a superset of Javascript,.
* the repo is selfcontained, "npm install" will install webpack and all tools you need to build the bundle.