It seems it is only for offline tables:
https://docs.pinot.apache.org/operators/operating-pinot/minion-merge-rollup-task
The Minion merge/rollup task allows a user to
merge small segments into larger ones, through which Pinot can potentially benefit from improved disk storage and the query performance. For complete motivation and reasoning, please refer to the design doc above. Currently, we only support
OFFLINE table APPEND use cases.