Nothing on lemmy is private. Your instance is just hosted on a server, and in this instance that server is essentially just someone elses computer. Anything you do or say on the server can be viewed by the admin and whoever they decide to delegate access to.
This is true for practically every online service ever.
Legacy software with incredible backwards compatibility, exponetially more software options, user familiarity, pretty much everything that active directory provides from user management to group policies, the list goes on.
Im a linux guy, but the thought of rolling out even the most user friendly linux distro gives me nightmares.