The trick shown in the linked post is rather simple: you only need a single paragraph, left-aligned.
Then make sure there is only one tabstop defined, a right-aligned tabstop at the right edge of the cell.
To the paragraph, add the text you want left-aligned, then add a tabstop, then add the text you want right-aligned.
Sample code:
var table = section.AddTable();
table.AddColumn("8cm");
table.AddColumn("8cm");
var row = table.AddRow();
var paragraph = row.Cells[0].AddParagraph("Left text");
paragraph.AddTab();
paragraph.AddText("Right text");
paragraph.Format.ClearAll();
// TabStop at column width minus inner margins and borders:
paragraph.Format.AddTabStop("7.7cm", TabAlignment.Right);
row.Cells[1].AddParagraph("Second column");
table.Borders.Width = 1;