I followed these instructions in Confluence and got a calendar url of
https://confluence.cms.gov/rest/calendar-services/1.0/calendar/export/subcalendar/[calendar ID redacted].ics?os_authType=basic&isSubscribe=true
Note that when I follow that URL in a browser, I get a .ics file that downloads, and when I open it in Mac OS Calendar app, it imports the events as expected. That URL requires authentication though – if I visit it from an Incognito window, it prompts for a login.
I copied that URL, and in Google Calendar, I went to “Add Other Calendars” > “From URL”, and pasted in that URL. None of this Confluence calendar’s events show up in Google Calendar, and there is no prompt in Google Calendar to enter a username or password. The added calendar is set to be shown in Google Calendar (the checkbox next to it in the list is checked).
I referenced this similar question: How can I get my TripIt calendar to show up on Google Calendar? and saw that it said Google Calendar can take up to 24 hours to refresh events from an external calendar, but I first added this calendar over a week ago, and I am still seeing no events in Google Calendar. None of the other suggestions resolved my problem.