If a dashboard variable called resource is created, it can be referenced as ${resource} within Grafana, and there are some formatters that Grafana can apply to modify the replacement string. The fields in the query editor expect variables in $resource notation and currently use the exact string ${resource} instead of handling the replacement with formatting options.

See https://grafana.com/docs/grafana/latest/dashboards/variables/variable-syntax/ for details on Grafana variable syntax.