The BFF pattern is just "mostly microservices dedicated to a particular client type".
It makes sense when you have drastically different needs between a desktop client and a mobile client (or maybe for a kiosk client or POS interface)
Hosting a microservice is cheap, it avoids unnecessary workload on backend data stores, and teams can operate with more autonomy if they don't have to cooperatively update APIs in coordination with other groups with differing priorities.
This article really just reads like "I figured out how to do authentication with keycloak using OIDC"