As of the current support page, only "public" calendars are allowed:
You can only add a calendar using a link if the other person's calendar is public.
(The support page is about adding "someone else's Google calendar" specifically, but it seems to apply for any iCalendar source as well.)
You can get around this by:
- Asking the calendar's owner to publish the calendar publicly. It can be a separate URL or the same one (DAViCal for example keeps requiring auth on the editable calendar URL & makes it available without auth on a separate one). Adding a secret to the URL can help to prevent guessing it. In many cases (eg. by publishing with DAViCal) the calendar author can still control each event's visibility (show completely, show only date/time but not details, hide it completely in public calendar).
- Use some proxy service (as suggested by @666damo) to publish a private calendar (requiring auth) on a new public URL (which doesn't require auth).
To clarify, even if a URL containing basic auth (like the one in the question's example) works in other places, it doesn't work in Google Calendar when You try Add other calendars > Import from URL. In my recent test it doesn't even notify the user about the failure.