Is this accurate? I don't have a lot of firsthand experience with data science stuff, but usually when looking just past surface-level you find that some Python package is really a shell around some native binary core implemented in e.g. C (or Fortran?).

When I at the repos for spaCy and its assistant Thinc, GitHub's language analysis shows that it's pretty much Python. Is there something lurking in the shadows that I'm not seeing? Or does this mean that if someone cloned spaCy and Thinc and wrote it in JS, then the subset of data scientists whose work can be done with those two packages (and whatever datavis generators they use) will benefit from the faster runtime and the the elimination of figging and other setup?

I call this implicit step zero.

Agreed. NodeJS is developer tooling. It's well past the time where we should have started packaging up apps/utilities that are written in JS so that they can run directly in* the browser—instead of shamelessly targeting NodeJS's non-standard APIs (on the off-chance everyone in your audience is a technical user and/or already has it installed).

This is exactly the crusade I've been on (intermittently) when I've had the resources (time/opportunity) to work on it.

Eliminate implicit step zero from software development. Make your projects' meta-tooling accessible to all potential contributors.

* And I do mean "in the browser"—not "on a server somewhere that you are able to use your browser to access, à la modern SaaS/PaaS"

... assuming you're a Rubyist and you have gem and ruby installed.

That's the explicit goal of triplescripts.org in its early stage—eliminating "implicit step 0" from software development.