Appending ("Col A" "Col B" "Col C") to (("Row 2") ("Row 1")) with gnc:html-table-append-column! creates (("Col C") ("Row 2" "Col B") ("Row 1" "Col A")), which is correct. But: (gnc:html-table-get-cell test-table 2 0) "Col C") leads to an error: "Value out of range"
A solution proposal is covered in PR#391 see https://github.com/Gnucash/gnucash/pull/391/commits/e44f2c475ea92c3d33866cd263ba9bb9c74e6aaa
Which is merged and will be released in GnuCash 3.5.