Oh I agree, generic run anywhere platforms are crap. This is only run on windows. Although it could run on other platforms with work, that was not the requirement so we took advantage of some native windows features.
The UI is very separate from the back-end. It does a lot of the data manipulation/creation with database code. Returned spatial features are also cached on the middle tier to enhance scale-ability. But the brute force editing and rendering of map data for both vector and raster is done on each client. To do that on the middle tier frankly would not work due to bandwidth restrictions and latency. I'm talking max 3G speed network connections (the pathetic DoD NIPRNET), so long-term caching of data on the client is paramount. Not to mention local printing to large format printers (aka plotters), multiple P2P connections from a client to multiple data sources (e.g. WFS, WMS, network shared shapefiles), etc. Just the P2P bit rules our using a browser due to the cross-scrip vulnerability crap, or a mix of HTTP / HTTPS data sources.
But back to "app" platforms for desktop / tablet OS's - the choice now is essentially compiled, locally installed and updated .net apps or whatever - or - Java, with JWS as a sort of hybrid between that locked-down environment and thin HTML apps.If there was a better, well supported hybrid environment, we'd be on it.