Overwrite columns

In that case you need to merge your table (with comments) and the information from Sharepoint in a query which sends it output back to your table (which serves both as input and as output).

In order to do that, you need to have a key column (or a set of key columns) you can use to merge both tables.

So your query structure is like:

get table from Sharepoint

merge with Excel table and expand (only your comment column so the comment will be added to the table from Sharepoint)

And the output of this query is written to the Excel table.

Edit: example of a working query how it looks like in the end.

It requires some steps to set it up, as illustrated in the video that I linked in a previous post.
The name of the query is ExcelTable, so the output is used as input with the next refresh.

let
    Source = SharepointTable,
    TableWithComments = Excel.CurrentWorkbook(){[Name="ExcelTable"]}[Content],
    #"Merged Queries" = Table.NestedJoin(Source,{"Header 1"},TableWithComments,{"Header 1"},"NewColumn",JoinKind.LeftOuter),
    #"Expanded NewColumn" = Table.ExpandTableColumn(#"Merged Queries", "NewColumn", {"Comments"}, {"Comments"})
in
    #"Expanded NewColumn"