That's not iOS fault. Apps can store their files in a folder visible in the Files app, or can ask the user to open a file or folder from a file provider (also visible in Files app), or to save a file or folder in a file provider (always visible in the Files app).
It's not the 2011 iOS anymore, if an app today hides its video projects from the user, it's entirely the app fault.
Arguably this is still on Apple, because they don’t let you access the full filesystem as you can on other operating systems, and in particular because an app developer may rightfully want to create a class of internal-use files that are not explicitly exposed to the typical user, but would be available to users seeking them out.
I imagine, for example, that if the internal project files for a popular video editing app were accessible, we’d see competing and/or open source apps emerge that could parse them, were the original app to become suddenly unavailable. Instead they’re just lost because your phone won’t let you access them.
Blame can be shared. The OS vendor for providing a way for applications to hide files on the user's filesystem from the user, and the application for using it instead of making the user's documents available to the user. They are both working together in unison against the user.
imo it's the platform's choice to have default-visible or default-sandboxed program outputs and data.
while possible, it is fairly non-trivial for iOS apps to have read/write access to a shared folder where they can drop arbitrary files, which can then be accessed by other apps, or be discovered by the user. it often requires copious permission negotiation handling codepaths by the developer, and a fearlessness of scary permission-warning dialogs by the end-user.
even on modern (commercially popular flavors of) Android which no longer imbibes the "free software" ethos of the linux core the OS was built around, you can't access formerly accessible application sandbox folders without installing third party browsing tools or plugging into a desktop computer to mount the storage, and cross-application sandbox access is similar to iOS.
in the "personal computing way" mentioned by the article (even today on desktop environments, less so on MacOS) program outputs are default-visible, and developers have to go out of their way to firewall or obscure or encrypt it from being accessible by the user or other programs using OS-provided pathways.
i think this is 100% on the OS + hardware + application platform provider (with Apple as all three on iOS).
It's not the 2011 iOS anymore, if an app today hides its video projects from the user, it's entirely the app fault.