I don't see anyone saying to use --wpfc and -wpredis on the same site.
That's exactly why rexi88 said to create one site with --wpfc and another site with --wpredis that you later delete.
Conversely, kingkool68 said to use the ee stack install --redi/ee stack install --phpredisadmin commands after you've created a site with --wpfc.
Still seeking clarification on Nginx Helper and the missing Key Prefix in the Redis plugin though.