I believe the authority stems from the gameplay mechanics that indicate the Delegate as leader of TWP. The Guardians likely had some input on the treaties as their influence is also a factor. To be fair, the Delegate doesn't ever need to consider our input on any issue. If the forum community gets unruly, it can be displaced.
That said, the forum based government has shifted away from game-side interactions and is designated as the government of those deciding to take part on the forum, not as a representative body of the region as a whole. Therefore extra-regional treaties would not influence us here directly.
Likewise, barring recent legislative endeavors, the military has maintained its structure separate from the HGA and while it utilizes these same forums for organization it is not (again, unless recent legislation has been approved by the Delegate making it so) beholden to the HGA directly.
The intelligence gathering aspect of TWP is wholly outside of the HGA I believe.
You may have a point in regards to the cultural components, but those generally involve mundane activities like joint celebrations, which are hardly impactful on our ability to maintain our own internal cultural identity and therefore not likely to require HGA approval.