It's probably an assumption based on the fact that he's based on Lucifer. But officially, there's no ranks within the Demon Lords. Lucemon FM was merely the one who instigated the others, but he never actually led them beyond that first suggestion.
And, no DigiWiki isn't reliable at all. Beelzemon being the strongest of the Nightmare Soldiers? Ha! Ogudumon, Millenniumon, and Leviamon (who is officially the strongest Demon Lord and a member of the Nightmare Soldiers family) would love to hear that. Daemon "Lord Mode"? What a bunch of crap! That's just as bad as the Sakuyamon "Kabuki Mode" garbage that was floating around last year. The people who edit that site often make up whatever they want based on what they think or like to believe and if we're gonna be like that, then Barbamon should be the leader because his beard is awesomely manly and because I say so and it'd be every bit as valid as their point. Anyway, try Wikimon instead: http://wikimon.net/Main_Page
It's still got its flaws, like any wiki, but at least this one isn't nearly as off as DigiWiki and has more official sources.
"Daemon "Lord Mode"? What a bunch of crap! That's just as bad as the Sakuyamon "Kabuki Mode" garbage"
While I may agree to some degree on this(they arnt different modes just different costumes), they do exist. One is just Daemon in a robe (see season 2 of the anime) the other is Sakuyamon in a preistess atire (I forget what the Japanese name for it is)