Prevent Google spreadsheet editor from accessing GAS by using detached GAS

The solution is sketched in this post, but it seems key steps are not identified.This code works fine if in GAS bound to the spreadsheet:

function onEdit(e) {   var sheet      = e.source.getActiveSheet();   var activeCell = sheet.getActiveCell();   var row        = activeCell.getRow();   if(row != 1) {     sheet.getRange(row, 4).setValue(new Date().toISOString())    }}

In the owner account's G-drive, I created a script as follows:

function onEdit(e) {   var ss         = SpreadsheetApp.OpenFileById("[spreadsheet ID]");   var sheet      = ss.getActiveSheet();   var activeCell = sheet.getActiveCell();   var row        = activeCell.getRow();   if(row != 1) {     sheet.getRange(row, 4).setValue(new Date().toISOString())    }}

But this script does not set the date value in the active sheet when an edit is made. I deployed this detached script (as webapp) and gave it permission to access the spreadsheet. But I suspect there are other steps or I have not properly deployed the script so that it can respond to edits in the active sheet of the spreadsheet.