The primary filter in all of our tanks is the bio filter provided by our live rock and live sand. Protein skimmers are another form of filtration, as are various reactors, floss, filter socks, etc. HOB filters offer the option to run floss, carbon, GFO, etc. on sumpless/reactor-less tanks. While a protein skimmer in tandem with this would certainly be recommended, claiming that a HOB can't run a 20-30g reef tank just fine is ill-informed. I actually have less filtration in my Solana 34g... I run a Tunze 9002 protein skimmer and I have a bag each of carbon and GFO sitting passively in the back chamber. I don't even have floss in my set-up. I grow everything from softies to sps just fine with this set up.
I'd be curious to know what type of filter you'd recommend instead. Sumps don't automatically create filtration in and of themselves; they simply allow for greater water volume and for the hiding of reactors etc.