That link is for Excel. Excel sheets don't have recordsets.
My example referenced RecordSource property, not a field name. But did not test and example was incomplete.
If subform RecordSource is a simple SQL statement SELECT * FROM table2; following code will build an SQL statement.
Subform1.Form.RecordSource = "SELECT * FROM table1 WHERE NOT ID IN (SELECT MatchNum FROM (" & Replace(Me.subform2.Form.RecordSource, ";", "") & "));"
If subform RecordSource is simply a table or query name:
"SELECT * FROM table1 WHERE NOT ID IN (SELECT MatchNum FROM (SELECT * FROM " & Me.subform2.Form.RecordSource & "));"
Nested subquery serves as list. Including subform filter criteria will get a little more complicated. How is subform filtered?
If you want to provide db for analysis, follow instructions at bottom of my post.