I think that was because they were making a 5-sided RTS game, and because four were already filled (Fed, Kli, Rom, Car) but there wasn't really mention of a 'big empire'-ish thing to match them, well, Ferengi were up because TNG said they were a mysterious power, so although DS9 said they were after money, it automatically became, "So... Ferengi is like Capitolist America on steroids?! America = Capitalists, also America has a big army, navy and air force etc... so... Ferengi = Capitalists = America which has a large military, means Ferengi = empire with a large empire"... and since *ds9* stated the head of the Ferengi state is a Grand Nagus...
yeah they were stretching things, but the ferengi as an organization themselves weren't a creation of DS9 that got crossed over to TNG, but the other way around. Section 31 as an organization WAS a creation of ds9, that never crossed over.
Honestly, I would have liked it if DS9 actually had gone "oh yeah the Ferengi actually are a power, smaller than the others but technologically on par, but Quark, Rom and Nog are like the outliers of their society, Ferengi as a rule were insular because other cultures weren't nearly as self-interested capitalists as they are, it's just that Quark ended up in cardassian territory rather than federation that resulted in Quarks' Bar being on Terok Nor, not Starbase 54 or whatever." because of course they are, why else would Gamma quadrant powers be interested in trading with the ferengi etc, if they themselves weren't a power to be ... bargained... with? eh? eh?
No?