We have investigated it bit more. Please use the following code:
Workbook workbook = new Workbook(srcFile);
foreach (Worksheet worksheet in workbook.Worksheets)
//as File > Print > Page Setup > Sheet > Comments > As displayed on sheet
worksheet.PageSetup.PrintComments = PrintCommentsType.PrintInPlace;
//As descirbed in https://support.office.com/en-us/article/the-difference-between-threaded-comments-and-notes-75a51eec-4092-42ab-abf8-7669077b7be3
//Notes (formerly called "comments" in earlier versions of Excel) don't have a Reply box.
//as Review > Notes > Show All Notes foreach(Comment comment in worksheet.Comments)
//show all notes, not ThreadComment.
if (comment.ThreadedComments.Count <= 0)
comment.IsVisible = true;
var saveOptions = new PdfSaveOptions(SaveFormat.Pdf);
The key point is: Notes (formerly called “comments” in earlier versions of Excel) do not have a Reply box so we just set Notes (not include ThreadComment) to visible.