OK, I've finally figured this out.
I've been running intern using
.\node_modules\.bin\intern-client config=test/intern
while it should've been
.\node_modules\.bin\intern-runner config=test/intern
Thing is that intern-runner and intern-client are two different applications, one is for running with browsers via WebDriver, the other one for running with Node. It didn't catch my eye even though I was reading and re-reading the docs much more than once. Probably the distinction should be highlighted there.
Hope this helps someone )