In Google Calendar settings you click “make public” then below that is the “get sharable link” link. But, in true Google style, this isn’t the link to the publicly sharable calendar, from what I can make out it’s a link to share the calendar ONLY with other users of Google Calendar.
To get the proper public share link you have to scroll down a bit, to a completely random section and get the link there. Clear as mud!
I’ll leave this here in case others hit the same issue and search for the solution.