Actually, I think there is a difference, and that is also perahps wrong on the code from your link.
They mention the following code to change JPEG quality setting:
occ config:system:set jpeg_quality --value 60
whereas I believe from my research that the correct setting should be:
occ config:app:set preview jpeg_quality --value="60"
The reason for this is in the code snippet where you can see the Nextcloud is looking for the value jpeg_quality in the app setting for preview.
$quality = $this->config->getAppValue('preview', 'jpeg_quality', 90);