getting rollup working with rollup.config.ts

In this repo, I'd expect

npx rollup -c to work, or if not that, than npx rollup -c --configPlugin typescript

but neither do.

however, npx tsc && npx rollup -c rollup.config.js does work.

What am I doing wrong?

I have some scripts of exactly what I've run:

npm run tsc creates a valid rollup.config.js (see npm run rollup) but npm run rollup-ts does not work.