private void radButton1_Click(object sender, EventArgs e)
{
    GridViewSpreadExport export = new GridViewSpreadExport(this.radGridView1);
    export.ExportChildRowsGrouped = true;
    export.ExportVisualSettings = true;
    export.ExportHierarchy = true;
    var renderer = new SpreadExportRenderer();
    renderer.WorkbookCreated += this.Renderer_WorkbookCreated;
    export.RunExport(@"..\..\Exported\spreadsheet" + DateTime.Now.ToString("HH-mm-ss"), renderer);
}
 
private void Renderer_WorkbookCreated(object sender, WorkbookCreatedEventArgs e)
{
    var worksheet = e.Workbook.Worksheets[0];
    CellRange usedCellRange = worksheet.UsedCellRange;
    for (int rowIndex = usedCellRange.FromIndex.RowIndex; rowIndex <= usedCellRange.ToIndex.RowIndex; rowIndex++)
    {
        RangePropertyValue<int> outlineLevelRangeValue = worksheet.Rows[rowIndex].GetOutlineLevel();
        int outLineLevel = outlineLevelRangeValue.Value;
        if (outLineLevel > 0)
        {
            RowSelection rowSelection = worksheet.Rows[rowIndex];
            rowSelection.SetHidden(true);
        }
    }
}

I have also updated your Telerik points.