That picture shows the complete monitoring stack with lots of optional components (pushgw, service discovery, Grafana).

When comparing to OnCall you need OnCall AND still the rest of that Prometheus picture.

Compare with this picture where everything in the leftmost Alert Detection box is what you see in the Prometheus architecture. https://grafana.com/docs/oncall/latest/getting-started/

OnCall also does nothing unless you have something external firing alerts for you. They both fill similar niches in a larger monitoring system; this does not excuse OnCall having a drastically more complex internal architecture.