Moon busting is eons of times less impressive than destroying the earth, so him being above moon busters doesn’t mean much There is no proof he can do more than destroy the planet with Gallick Gun, hell wasn’t the requirement to do so having around 20 000 PL ? Which is where Vegeta was iirc.
Here’s the thing though — Piccolo fighting Raditz has a power level of 408. When he was training Gohan, it was no more than a few weeks into their session since this was the first moon Gohan saw in the wilderness.
Meaning Piccolo wasn’t
drastically stronger when he destroyed the moon than when he fought Raditz
Raditz has a Power Level of 1,500. He’s almost 4x as strong as someone who could casually moon bust.
Enraged Gohan was clocked in at 2,800 — Still did shit to Nappa whose power level was ambiguous and was established as an insect in front of Vegeta.
Vegeta’s canon power level is also 18,000 during this fight (180,000 in Oozaru). So he’s 44.117 times as strong as the casual moon buster..
Yeah, I’m locking him as a casual planet buster given the insane leap in power there. Never heard of the 20k rule tho.