Google's instructions are not only inconsistent but incorrect in some cases. I have been using gmail since 2005 or 2006, got my gmail address, and later added an alternate address from my German GMX domain to my google account.
When I tried to follow the instructions, I found out that there is no such option "allow responding to invitations forwarded through alternative email addresses" in my settings.
To solve the problem, I have filed a Google calendar support issue here:
How to prevent 404 error in gsuite calendar when accepting ics invitation sent to my alternate @gmx..
Please note that I have to post this as an answer, as I have no capability to post comments in the webapps stackexchange yet, so please consider this before downvoting. I plan to update my answer once my case will be successfully solved!
Update: Turns out one year later that there was another distinct Google account for my other email address that I must have created sometime. After deleting that other account, it was possible to add the alternative email so that I can now fully use it for Google calendar.