It was explained to you by @Huginn that the linked article simply explains how the basic Discourse software works. The information contained does NOT apply to specific applications such as this forum.
Like most software, Discourse can be customized to whatever level is required, so the settings of this forum are set beyond the basics of 15 days.
Instead of worrying so much about being able to PM, why not actually start contributing to the community? Putting so much emphasis on private messaging is highly suspicious behavior and may lead to being flagged for the moderators.