The reason we request the broader permission is it better interoperates with the cloud storage for browsing and sharing files. The research we originally did was a couple of years ago now and I'm not sure if things have changed, but the app-folder restricted permission meant it only accessed an invisible folder unique to the app - so you couldn't even find your files by browsing the files on the web (e.g. on the Google Drive website). I think it would also make it more difficult or perhaps impossible to support other sharing features, such as the ability to browse folders shared with you, and load project files from those (which is particularly important for some customers like in education).
I can assure you Construct only accesses the files you specifically choose, and solely for the purposes of opening and saving the projects you tell it to. We've also been through the security reviews that Google and Microsoft require for using these permissions.
The one exception to this is Dropbox, which does support an app folder that you can also access on the web. So for Dropbox we do only request permission to access a special app-specific folder, and it can't see any files outside of that. So if you want to choose a cloud service that requires least permission, you could go with that.