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.
First off, want to tackle him being the most powerful of the Dark Masters. While this is true, the other Dark Masters weren't exactly what I would consider upper tier Megas. They were all beaten by Metalgarurumon and Wargreymon and Piedmon himself was defeated with a relative amount of ease by MagnaAngemon, all of whom are far weaker than the 4 holy beasts.
The being comparable to the four holy beast is a bit of a question. From my understanding it's a bit of a question on how much they were amped by Apocalymon to accomplish sealing the beasts as if they were more powerful they more than likely would've just killed them. As stated before, we see Piedmon lose quite handily to MagnaAngemon and wasn't able to keep up with a WarGreymon and MetalGarurumon who knew what they were doing.
I looked at the page where you got that from and find a lot of it questionable to say the least and seems to be operating under an understanding of the digital world(s) that does not line up with any of my experiences. In all of the shows I've watched (seasons 1-4) and games I've played, each instance of the digital world and earth are treated as their own separate part of the multiverse with no real connections to each other much in-line with how Earth 616 lines up with Earth 1610 and having similar people and characters but are completely separate iterations. As per Piedmon himself, he's usually just treated as a strong mega level digimon (a level 6 and not the level 7 that Omnimon and such are at). In Survive, for example, he is capable of keeping up with Takuma's partner at at tier 6, but is ultimately humiliated by the end of the battle. He's an inconsistently powerful character and shouldn't be looked at only with high end stuff when he has plenty of low ends to balance it out. I also noticed that page kept on referring to him as a reality warper which isn't really true per say. He's got his keychain thing he does and the Masks Square ability, but the latter is more of a pocket dimension where he can teleport himself and his attacks wherever he wants within it (plus reading communications). Sorry if that's a little rambley.
He also can't even travel to our world (as a wizard digimon he might be capable of dimensional travel though we never see it.)