Yeah you’re wrong. The “Files” app on iOS, which is also embedded in various apps as a file open/save/import/export/share/etc option, has a plugin architecture where third party apps can provide all the same file storage as iCloud

Photos, contacts, messages etc aren’t exposed to Files. The person you’ve replied to seems to be talking about cloud-syncing them with a third-party service or backing them up in a computer-decryptable way.

you can sideload enterprise/school/work related apps

But any other personal app will not be downloadable unless you plan to only use 3 that aren’t already installed.