Grammatically speaking, C11.3 does not authorize Guns to start hooked up. It merely describes the effect on Emplacement of a Gun starting hooked up. That is, C11.3 assumes it is possible for a Gun to start hooked up, without explaining how that might come to pass (for example, by SSR only?).
That said, I think the place to find the permission to start hooked up is A2.52 Offboard Actions. It states, about units starting offboard: "SW/ordnance may start dismantled/limbered, and an AFV may start CE or BU. Vehicles may start loaded up to their normal capacity and are assumed to be in Motion (D2.4)."
More to the present point, it then says: "Units setting up on-board have these same options except that on-board vehicles may not set up in Motion unless so stated by SSR."
It would have better if 2.52 had stated "SW/ordnance may start dismantled/limbered/hooked-up." But the absurdity of off-board vehicles required to start in Motion, but without any Gun they have available not hooked up leads me to believe "hooked up" has to be there (in "loaded up to their normal capacity" perhaps). Particularly when the only action permitted to be undertaken off-board is to Deploy, so if off-board vehicles can't start hooked up they'll have to leave the Gun off-board or leave it to the crew to push it onto the field of battle!
