I just created a quickie simple iAssemblyFactory type assembly document, then did some trial/error testing on this, and got it to work for me. It was the file name extension that was tripping us up. We're both expecting it to be an ".xlsx", because that's what all the newer regular Excel documents put out, but it needed to be the old ".xls" file extension for me, for it to work, even though I'm using the 2019+ versions of MS Office & Excel. Quirky.
My working code looks similar to your initial code, but just adds the ".xls" file extension.
Dim oADoc As AssemblyDocument = ThisDoc.Document
If oADoc.ComponentDefinition.IsiAssemblyFactory Then
Dim oPath As String = "\\DIV29-FS-01\Shared\Engineering\SHARED\Tests\iAssemblyTableExported.xls"
oADoc.ComponentDefinition.iAssemblyFactory.Export(oPath, FileFormatEnum.kMicrosoftExcelFormat)
End If
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Wesley Crihfield

(Not an Autodesk Employee)