Good question. You gotta see which force is acting against that pressure. The telescope is not at the L2 exactly, but slightly “before” so it has the ever slightest tendency to get pulled towards the sun. This gravitational force is bigger than the solar wind pressure, so you need fuel to ever stay as close to L2 as possible, not overshot.
The aft momentum flap is to counteract rotation caused by the wind pressure, and not to act against the outward force of the wind pressure: the shield is rarely perpendicular to the sun rays, which causes a rotation as the sun beam is reflected off.
First, solar light has many many many times more effect than solar wind. Then, the shield acts as a light sail, but it’s rather small force (at this distance from the Sun the force is about 1/100th pound per acre or about 1kg per square kilometer) but it’s accounted for in orbital positioning (the orbit would be slightly shifted towards the Sun to counter that).
The main effect is that solar radiation would try to rotate the craft to “topple” it, as the mass behind the shield isn’t symmetrically distributed. But this is accounted for too, as the craft has special solar radiation “rudder” which allows countering this.
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