Hi,
Did you run that command line with administrator privilege?
I tried DISM export command on my side, all of the default app file associations almost 100 have been exported on existing account.
dism /online /Export-DefaultAppAssociations:"%UserProfile%\Desktop\MyDefaultAppAssociations.xml"
Do you want to export a special app file association as I noticed you said none of which you interested in?
Bests,
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