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).