About Files app not being "single file" because it loads JS/CSS resources from the CDN (internet). That is of course a correct observation, and this is already noted on the website. From an operational perspective, there is only one file to download/move/install, and from a functional perspective for the vast majority, it will be viewed in browser with an active internet connection. Furthermore, it was INTENDED BY DESIGN to load all JS/CSS from CDN, because ultimately it improves the user-experience, and makes it plausible to make an app like this wrapped into a single index.php file.
It's not reasonable to add ALL Javascript/CSS directly into the index.php file. PHP is only 10% of the codebase, and it would bloat the file to insane dimensions and make it difficult to maintain. JS/CSS is loaded from CDN service jsdelivr.com, with multi-level redundancy (Amazon, Cloudfront, Cloudflare), and will load faster and cache more effectively than anything served from within the index.php app on your own server. Furthermore, some plugins are served on-demand, for example when browser needs a "polyfill" or for specific features. For functional reasons, benefiting the vast majority who do not care how/where the JS/CSS is loaded from, JS and CSS files are therefore served from CDN. If this does not suit you, nobody has any problem with that.
Does anyone have any examples of any other single-file app with a comprehensive interface like Files app that is entirely self-contained?
There will be a future release where you can assign all scripts/css to be loaded from local path, but then of course all these files will need to be installed alongside Files app index.php. We will NOT be adding all JS/CSS directly into the index.php file.
About license, as stated on the website, you can use it as a file viewer for free (with the nag). If you want to remove the nag and use it as a file manager, license is required. Those who expect apps like this to be entirely free with updates and support, are living in dream land. It's hardly worthwhile in the first place. There are loads of other free apps, and if you can't see any difference, then your choice is easy.
I have noted comments about license info being unclear, thanks.