Correct. In your itch.io project page, under Visibility & access make sure you have selected Restricted . This will prevent it appearing in itch.io searches and what have you, which would be a public release. You can set it to Public about 24 hours before submissions close. Perhaps set a reminder in your calendar.
I'll send an email reminder to all participants about 24 hours beforehand. If you forget, it doesn't matter. You'll know, because your game won't appear on the Submissions page after submissions close. If there is a mismatch between the number of entries and the number of games that appear, this is usually the reason why. You can then change it to Public and it will appear. So long as it's submitted before the deadline, you're safe.