I think that's about right, but the real purpose isn't resolving spatial details on the planet (which you correctly found isn't possible), but to find the plane

I think that's about right, but the real purpose isn't resolving spatial details on the planet (which you correctly found isn't possible), but to find the planet at all—to distinguish a point of light out of background noise. A fine angular resolution is still helpful, for SNR reasons: smaller pixels contain less noise, wheras the signal pixel contains the same amount of signal!