I seems that you are using some older version of Aspose.Cells. I tested your scenario using a simple template excel file and it works fine as the conditional formattings is set to my defined cell area with the reference cell formula1.
Kindly try the attached version (18.104.22.168).
//Instantiate Workbook object
Workbook workbook = new Workbook();
Worksheet sheet = workbook.Worksheets;
//Create an empty conditional formatting in the first worksheet
ConditionalFormattings cfs = sheet.ConditionalFormattings;
int index = cfs.Add();
FormatConditions fcs = cfs[index];
//Set the conditional format range.
CellArea ca = new CellArea();
ca.StartRow = 0;
ca.EndRow = 10;
ca.StartColumn = 2;
ca.EndColumn = 2;
//Add condition. since A1 cell has some integer value.
int conditionIndex = fcs.AddCondition(FormatConditionType.CellValue, OperatorType.LessThan, "=$A$1",null);
//Set the Font text color.
FormatCondition fc = fcs[conditionIndex];
fc.Style.Font.Color = Color.Red;
fc.Style.Font.IsBold = true;
//Save the Excel file