Hello, from my understanding this workaround isn't equivalent from a security point of view. Ability to use Docker Swarm secrets is a must have for me. I'd really like to see this security feature in PiHole v6 as it worked in v5 please.
Other breaking changes about using PiHole in Docker were really well documented and this went smoothly for me so a big thanks for your work 
Best regards
Other references :
https://github.com/pi-hole/docker-pi-hole/pull/584
https://github.com/pi-hole/docker-pi-hole/issues/556