First of all, I'm in disagreement with the reasoning at the top and bottom of the OP. First of all, calculated feats > Author statements. A power level of 10,000 being a requirement to destroy a planet is very contradictory since planets can vary in size, and planet busting feats are often way above planet level for obvious reasons. Also, Vegeta is clearly more durable that the potency of his Galick Gun; although did match Kaioken x3 Kamehamaha, Vegeta still tanked the Kaioken X4 Kamahameha and was just fine. Galick Gun being able to bust the planet scaring the cast has less to do with the idea of a 5-B character and more to do with Vegeta simply being strong enough to kill everyone with Goku being helpless to stop him. Moon busting feats can often range from 5-C, Low 5-B, 5-B, 5-A, and High 5-A. Similar to how Earth busting feats can vary from 5-B, 5-A, High 5-A and Low 4-C. Plus, Goku and Piccolo are obviously way stronger than they were in Dragon Ball saga.
As for the moon busting feat itself, nothing suggests that he simply vaporizes it; ki blasts typically nuke and blow stuff up rather than simply boil something till it because a gas planet. In fact, it's consistent with the "Shatter the moon" statements. Also, Kepekley did also calculate Master Roshi's outlier would get the same results; 5-B and Relativistic. I know his example was in outlier because it was back then and was via an attack that required every ounce of energy. But this is much later and more regular.
I'm neutral about perhaps some slight details being off, but I heavily disagree with just downgrading it all the way to 5-C. There's also the fact that Piccolo's Ki would also have to fight gravity to reach the moon among other things.