flarum-ext- is a prefix which has nothing to do with the vendor. It just makes it easy to tell on GitHub/Packagist that the package is a Flarum extension. So the example you've given isn't possible, because then the vendor would be omitted from the extensions directory name. To illustrate, look at a third-party extension:
tobscure/flarum-ext-example
extensions/flarum-ext-example
tobscure isn't present in the extension directory name so you'll get conflicts if there are two extensions with the same name by different vendors. Instead we could do:
extensions/tobscure-flarum-ext-example
But there's really no point in having the flarum-ext- prefix in the directory name, because we already know that it's a Flarum extension! It just makes the directory names unnecessarily long. So we've ended up with:
extensions/tobscure-example
We initially steered away from having another level of directories for the vendor (e.g. extensions/tobscure/flarum-ext-example... or even just keeping them in the vendor directory) because (1) it would've made extensions harder to upload/remove via FTP and (2) we need to be able to scan the extensions directory to list extensions, and then load their bootstrap.php files on every request.
