Remarkable's chrome extension does the same, no? And I have full control in the print dialog to adjust the output.

There's a value if you provide a CLI tool for that, for automation.

Looks really interesting, am I missing the code on GitHub?

The github link on the site takes you to your personal github page, not the page for the sendreadable project. One downside of the way remarkable handles this sort of thing, is that I assume I have to give you my API key, which would let you read/write anything you want to the tablet. Its a lot easier to trust that you're not doing anything fishy if you provide the code to the project.