How to remove timestamp or batch edit all comments

There's no reason what was described there shouldn't still work, but only for the properties you can edit directly, such as Author.

What you described can be done using a script, but only for the Modification Date property, as the Creation Date one is read-only.

To do that you can use the following code (from the JS Console, a Custom Command or even a button in the file itself):

var now = new Date();
this.syncAnnotScan();
var annots = this.getAnnots();
if (annots!=null) {
	for (var i in annots) {
		var annot = annots[i];
		annot.setProps({modDate: now});
	}
}