These are actually just plain old category pages to which they have applied some custom styling. It seems you've already figured this much out. This is the more correct approach IMHO when you have an unlimited number of stores and an unknown size of catalog, especially if you're doing something like a marketplace (think Zazzle) and have no budget or desire to get into marketplace extensions.
How'd they do it?
There are sub-categories in those categories which give each 'microsite' the ability to have some distinguishing taxonomy from another microsite (see the left nav in particular for filter/subcats).
Because of the way Magento gives you the ability to edit description fields and add static block mastheads to the top of categories you can really achieve some very interesting visual design which will further distinguish each category/microsite from another.
Use of sessions shouldn't be necessary so long as you're using only one store. You'll get a unified cart across all sites for free by virtue of the site being a singular store. If you're trying to create a cohesive feel from microsite categories to a branded cart to eventual checkout and conversion you will probably want to look into rendering your site's theme based on body classes and css rules rather than by sessions, cookies. You can alter the body classes in layout based on cart contents and this would be my preferred method to handle it.
However, the multi-store model is not terribly inefficient and would require for you to have hundreds of thousands of products before it becomes unweildy.
It may be the more proper way to go should any of your microsites ever need i18n or a dedicated mobile site or you wish to do theming the proper way and not as I described above.
Best of luck.