Datasets aren't directly mutable.
You need to change your script to something like:
self.props.data = system.dataset.setValue(self.props.data, event.row, event.column, event.value)
Or change your query return type to JSON so that the table data is "exploded" into a JSON like format this is directly mutable.
Note that doing either of those things will still only update the local data. If you want to update the actual table in your database, you need a more involved script; you should run an UPDATE query from onEditCellCommit and then refresh("props.data") once you've issue the update.