If you wish to use external Cron services, then yes, the WP-Cron has to be disabled so that line in the wp-config.php is correct.
And please also disable the WP Cron on the MainWP Dashboard > Settings > Advanced Settings page.
The instructions now say that I have to create different cron jobs if I don’t use wp-cron.php.
It says that I should call up the same cron job twice, once every minute and once every hour.
The problem on a little-visited site is actually only that the wp-cron.ph is not called up enough if I understood it correctly.
But I made a call to wp-cron.php via a cron job service page every minute (or every 5 minutes). Isn’t that really enough? If I use these CronJobs, I then have to deactivate the disablw wp-cron call in wp-config.php, then everything should work fine, right?
When the WP-Cron is disabled (on MainWP Dashboard > Settings > Advanced Settings page and in wp-config.php), you cannot really make calls to it.
As that help article instructs, you have to create separate commands for cron tasks you wish to run. For example, you can create only ones for Check for available updates and for Ping Child Sites and not for any others.
The important thing to realize is the need to create individual commands for individual tasks.