Shops usually overstock their tanks hugely. If you go to one which actually knows what they are doing, you'll often see huge filters (sometimes custom made) hidden under the tanks. The water quality in fish stores is not always great either which is why you should never just empty it in to your own tank.
If you want to FULLY cycle a tank (as in grow nitrate eating bacteria as well) you need a big filter and lots of media. The right kind of media too as it won't grow on everything, sponge will never reach that stage.
Almost all of the useful bacteria is inside the filter, not the tank itself.
Your right many shops do run centralised sumps but many don't which is why i refereed to an 'old school' shop that doesn't. Have a look at aquarium co-op online, you'll see 300 fish in a 29 gallon tank running off a single sponge filter and there is no ammonia or nitrite in the tank. Are you suggesting they 'don't know what they are doing'? You do not need massive filters for fish tanks, that is a fact. They do a lot of water changes but that's not for ammonia, it's for nitrate and waste organics generated by the massively overstocked holding tank.
Most bacteria isn't confined to the filter, its distributed across every surface in the tank and its entirely dependent on how much surface area you have in both. My large tank has a deep substrate, loads of rock and wood, plants etc so it will contain a significantly higher proportion than a bare tank which will contain next to nothing.
Nitrate eating bacteria also doesn't require a big filter and lots of media, it requires a low oxygen environment which is something completely different. That is almost impossible to do inside a commercially available canister filter regardless of what media you have in it. There is just far too much oxygenated water flowing over the media to achieve it.
To get nitrate eating bacteria you need to create an area of low flow and months of waiting for things to happen. Lots of reefers achieve this by creating a deep sand bed where the oxygen runs out. Another option is to do it in a sump where you can add baffles to exploit stratification and make the fresh water flow over the top so it doesn't doesn't mix with the deep bed of media underneath. You can achieve something similar in a freshwater tank doing the same thing but again you need a deep substrate or a low flow area but you need more than just a special media.
Media like biohome claims to be able to support anaerobic bacteria which I don't doubt but it needs to be in the right conditions and a fast flowing canister filter isn't that. It also doesn't replace water changes because it can't deal with waste organics and other toxins in the water.