I successfully imported an Outlook ICS calendar in the Google Calendar Web App (https://calendar.google.com). I even made the permissions so that "anyone" can see all event details, not just "me".
I can look at the calendar in the web interface, no problem. But what about the Android "Google Calendar" app? There it doesn't show up in the left flyout. My other Google events, reminders and holidays are all selectable. I already tried restarting my phone (Android 9), clearing the storage and cache of the Google Calendar app and re-syncing my Google account on my phone. Nothing helped.