Why an astronomer such as Moore thinks that there should have been stars brightly visible in the lunar sky, so much, that he thought they might even be visible in the solar corona. Et cetera, et cetera.
The solar corona is visible only when the photosphere (the really bright part) is obscured. On earth this happens only during a total eclipse of the sun; there's far too much atmospheric scattering when this happens at sunrise and sunset.
Total solar eclipses are so rare and so short that the ability to produce one almost on demand in space was and still is very scientifically useful. Hence the experiments to photograph the corona on many Apollo missions, and the "coronagraphs" that are important instruments on space-borne solar telescopes.
So Moore was asking if even the sun's corona was bright enough to obscure stars when the solar photosphere was obscured by the moon. He obviously knew that the sun itself was plenty bright to do that.
If Moore was an astronomer, it's quite likely that he saw a total eclipse himself and wanted to compare the crew's experience with his own. After all, the mechanism that let the crew view the corona was very similar to that of a total solar eclipse. In both cases the moon, lacking an atmosphere, did not scatter light from the photosphere around its limb, leaving the corona visible.
