The cheat sheet seems to be outdated. You need to specify a correct collectionId
to the command so it finds the correct database.
I managed to get an export done using:
$ pkcon install nemo-qml-plugin-contacts-qt5-tools
$ devel-su -p contacts-tool collections
# this will print something like:
# ID: qtcontacts:org.nemomobile.contacts.sqlite::xxxxxxxx31 Name: aggregate
# ID: qtcontacts:org.nemomobile.contacts.sqlite::xxxxxxxx32 Name: local
# ID: qtcontacts:org.nemomobile.contacts.sqlite::xxxxxxxx33 Name: SIM
# ID: qtcontacts:org.nemomobile.contacts.sqlite::xxxxxxxx34 Name: SIM
#
# now take one of the ids (full string) and export the contacts:
$ devel-su -p vcardconverter --export contacts_aggregate.vcf qtcontacts:org.nemomobile.contacts.sqlite::xxxxxxxx31
Or, putting it all together:
contacts-tool collections | awk '{print "vcardconverter --export " $2 "_" $4".vcf, $2"} | sh'