Short answer: the one-reply-per-thread cap is a Discourse trust-level setting, not something specific to this forum or to your account. Discourse ships with a new-user limit of one reply per topic (the site setting is newuser_max_replies_per_topic, default 1) for trust level 0. It stops applying once you reach trust level 1, which normally happens automatically from ordinary reading — Discourse’s default thresholds are entering around five topics, reading about thirty posts and spending roughly ten minutes on the site. Staff can also raise the setting or promote an account manually.

If it is blocking spec work right now, the legitimate route is to ask the staff directly: start a message to the moderators and admins from your logged-in account (the About page lists them and says to get in touch that way), or mail contact@activitypub.eu for anything urgent. Requesting trust level 1 for your account is an ordinary moderation request, not a favour.

One thing worth checking before you ask: silverpill noted you already managed a second comment in the FEP-302f thread, so the cap appears to bite per topic rather than across the whole forum. If so, that specific discussion can probably continue and the restriction will only show up again in each new thread.