why does the wing have to fold at all? If it rotates 90 degrees counter-clockwise, its "straight" edges (trailing in the original picture) now become landing surfaces. Truth is - I would see that as one more "thing" that can go wrong in battle to the extent that if the stabilizers (if that is what they are) would be locked into one setting or another - making it potentially difficult to change from its orientation back to where it should be.
Frankly? The only reason I can think of - for having surfaces like that, are for atmosphere-flow reasons (Ie control surfaces). It is one thing to be able to move in any direction, any "axis" in space by means of thrusters, but I have to wonder how effective those thrusters would be in an atmosphere? Traveller glosses over such design features entirely it seems.
Ever wonder who much force is required to get a vessel like the patrol cruiser to swap ends quickly? When dealing with 20 minute turns, quickly could be as slow as 1 minute. But when dogfighting in the atmosphere, quickly is going to have to be a wee bit faster than 60 seconds
Of course, there is perhaps one answer that might seem obvious...
Who said that the patrol cruiser in the illustration is flying right side up? In space, there is no down. What if the patrol cruiser in the picture is oriented such that its belly is "up" in the picture, and its topside is down? Instinctively, one would presume that the flatter surface is the down side, and the upside would be anything but flat - but one solution is to state simply that the illustration shows the ship flying with its belly upwards from the observer's point of view.
In all - a good question.
The more I look at the fins, the more I have to wonder. If you look at the starboard fin (ie right side of the ship assuming that it is right side up in the illustration), the fin has three lateral lines near its lower extremity. Any of those would be nice potential "fold points" if you will. But when you look at its opposing fin for the "interior facing side" (ie the side closest to the hull), you do not see those same three lines. The interior facing side of the fin shows itself to be relatively smooth in the sense that two of the horizontal lines are absent. That would imply that the center of the three lines is the only one that could potentially fold (in my opinion).
So, in the end, short of rotating rather than folding, I can't really see this fin design being functional. Hell, I'm having a hard time wrapping my mind around why it even exists.