The firing arc as defined by the rules reference:
FIRING ARC
A ship’s primary firing arc is the area formed by
extending the firing arc lines printed on the front of
the ship’s token. A firing arc extends across the play
area. A ship is inside a firing arc if any part of its
base falls inside the area formed by extending the
firing-arc lines.
• A ship’s primary firing arc is used for all attacks,
including secondary weapon attacks, unless
specified otherwise.
• A ship’s auxiliary firing arc cannot be used for
secondary weapon attacks.
Even secondary weapons use the primary firing arc so for the purpose of firing arc and TLT or dorsal turret, the primary firing arc is the firing arc for the weapon.
Q: If a ship has a turret primary weapon (or is equipped with
a secondary weapon), is the ship considered to have a
360-degree firing arc?
A: No. Turret primary and secondary weapons allow a ship to ignore its
printed firing arcs. A ship’s firing arcs are always the printed, shaded arcs
on its ship token.
Haven't you answered you're own question. Quote that you provided "A ships's firing arcs are always the printed, shaded arcs on its ship token"
The secondary firing arc is by the definition you gave a ship's firing arc. Unless you have something in the rules that says when it isn't an arc then it is.
The other rules say you can fire a torpedo from that arc. But nothing about it not being an arc.
Using your own logic and example it is an arc.
