This had nothing to do with the aircraft. The Swordfish had no special ability that would enable it to fly at night any better than any other naval aircraft of the period. This was an OPERATOR choice (kudos to the airmen who flew them) and excluding the use of radar
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All this talk about the Swordfish being able to operate at 100 knots, or 138 knots, fails to understand the secret of the Sworfish's success. The Swordfish was not succesful because of its speed. it was successful because of its lack of speed. and nowhere is that more impoprtant than understanding why it could operate in conditions at sea that would ground mist other aircraft.
Your right that it was not especially designed for rough weather flying, yet the design was such that it was well suited to such conditions.
The Swordfish was a large biplane, which for various reasons worked to its advantage in rough weather. Because it is single-engined it tends to look deceptively small from a distance and on photographs. By all accounts, Its fabric-covered metal construction was exceptionally sturdy, which is borne out by its low loss rates to non-operational causes. It was also given a good reputation viz reliability, but lacked refinement. The biplane wing had ailerons on both lower and upper planes, and leading edge slats on the upper wing, which gave it exceptional levels of control in high winds. For the take-off, the ailerons could be dropped 8 degrees to increase lift which again assisted it in rough weather when the fligt deckk might be wildly heaving and twisting. This coupled with the fact that approach speeds could be as low as 30 knots, made it the ideal rough weather aircraft. For storage on carriers, the wing folded backwards. In cold weather the open cockpit was uncomfortable, especially for the rear gunner. The wing struts, the robust fixed landing gear, and the generous size of wings and tail produced high drag, and the single 690hp Pegasus IIIM3 engine gave the aircraft a very modest performance. But it was reliable, which was especially important for the lonely night patrols over the arctic seas that were to be the task of the Swordfish. And its low performance and very low operating speeds coupled with its great strength and low loadings made it ideal for arctic condition. not by designm, as you rightly point out, but evident nevertheless.
The other aspect about the Swordfish that made it the ideal night strike and roiugh weather aircraft of its time is less measurable, but evident from all that i have read about the aircraft. By all accounts, the Swordfish was extremely easy to fly and easy to land on a carrier deck, a quality that would become very important for night operations on the small decks of escort carriers, and in the generally rough Atlantic and Arctic conditions. Although the Swordfish was stable around all axes, it could make remarkably short turns. It could also be dived vertically to very close to the sea surface, and then make an abrupt pull-out. Very little speed built up in the dive, and very liitle sppeed was needed to achieve lift. Therefore the Swordfish was not necessarily an easy prey for a fighter, but it was during the long, slow and straight run that was required to launch a torpedo, and operate in poor conditions and/or conditions of low visibility. These abailities stood it in god stead when operating in the ASW role....the Swordfish was the first aircraft to sink a U-Boat at sea, on the 24 November 1941, sinking U-454 off Tangiers ( based on the survivor statements, the U-Boat was surpised at being stalked by an aircraft that approached it from astern, as a closure rate of less than 50 knots)
The swordfish was the preferred kit in rough conditions. if you look at the operations north of the arctic circle (specifically the Murmansk runs), it was only ever undertaken by RN carriers, and the preferred strike aircraft was always the Swordfish even over other British types, for reasons relating substantially to the avove. The RN used all manner of aircraft types in the far north, both American and of British origin, but the real rough stuff that required constant flying in all conditions was almost exclusively done by Swordfish.