This is because groupColumn() defaults to the native 'count' aggregation.

column.treeAggregationFn = uiGridTreeBaseService.nativeAggregations()[uiGridGroupingConstants.aggregation.COUNT].aggregationFn;

The documentation for the treeCustomAggregations option says:

If a function defined here uses the same name as one of the native aggregations, this one will take precedence.