The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
1) While Cui may have been stronger than the Vegeta of the Saiyan Saga, that doesn't mean he had the power to destroy planets on his own. Vegeta's strength at that time was still far below what was needed to destroy planets.
2) There is no evidence in the Dragon Ball canon materials indicating that Cui had ever shown the power to destroy planets. He was clearly on a level far below the truly powerful warriors like Freeza and his henchmen.
3) Simply being stronger than a Vegeta from the Saiyan Saga, who cannot destroy planets, does not put Cui on the same level of power. There is a huge gulf between those who could just damage extensively and those who can actually destroy entire planets.
4) Cui has never faced any opponent of Omni-Man's level, so there is no way of knowing whether or not he could have withstood the latter's overwhelming power. Based solely on what has been shown within their respective universes, there is a huge difference in abilities between them.
5) In short, comparing Cui to a Vegeta from the Saiyan Saga, who was not capable of destroying planets, does not support the argument that Cui could have also accomplished such a feat. That is an extrapolation not based on the evidence of the canon materials.