this took way too long to figure out, but it is remarkably simple in hindsight, so here it is

in VBA in the main ThisDocument page put :

Option Explicit


Private Sub Document_Open()
CheckTime
End Sub

then, add a module, containing the following :

Option Explicit
Dim PreviousContent As String
Sub CheckTime()
PreviousContent = ActiveDocument.Content
Application.OnTime When:=Now + TimeValue("00:10:00"), _
Name:="DoCompare"
End Sub
Sub DoCompare()
If ActiveDocument.Content = PreviousContent Then
'if no changes after 10 minutes, save then close the document
ActiveDocument.Save
ActiveDocument.Close
Else
CheckTime
End If
End Sub

Even simpler is to check ActiveDocument.Saved: Document.Saved property (Word) | Microsoft Learn

A simplistic comparison of two text strings ignores the fact that the author may have spent time carefully applying formatting to the text. That effort would be lost.