Yes, It is true. When a user ubsubscribes from an email which was sent to him from a Pepipost account, he gets added to the suppression list of that account. As the user suppression list is maintained at an account level, these users will not receive any further emails from the same Pepipost account.
In order to fix this, It is advised to create a separate sub account for your transactional & Marketing/Promotional emails in order to track user actions separately and maintain such list suppression lists separately.