"No filesystem" does not mean "no ordering". In Plasma Active (not in Plasma Desktop, indeed, as milliams notes), the filesystem is not presented directly to the user. Rather, its file manager uses semantic information to order, present and search documents, images, etc. This semantic information can be its creation or modification date, but also tags you have assigned to it, or connection to a certain activity (which immediately makes your file easily accessible within that context, no searching even needed).
Of course you can still use a file browser and just go through your documents this way, but if you look at the Plasma Active UI as a whole, you will find that putting a "semantic" view central fits in very well (in fact much closer than many file system usages) with how you would naturally organise your "stuff".
The same features are also available in Plasma Desktop, where it serves as an add-on to traditional file system usage. You can for example connect any file to your current activity, and have files connected to this activity shown where you need them. You can even use a file manager to browse through your activities. It's a neat way to make multitasking saner, and if you don't use it, it doesn't get in the way.