“Free and open source software.” It’s an ethos that says that code should be free and open for people to use and improve as they see fit. The core of it is that if you modify any software that is FOSS, your software must also be FOSS. So overtime the software and what its used for improve, change, widen. Lucky for us, the movement has been ongoing for 50+ years, so it’s a mature ethos whose benefits are everywhere. Most of the internet runs on FOSS. Lemmy itself is FOSS.
It doesn’t necessarily mean an app is more private, but it does mean you can generally self host, as the commentor said. There isn’t a profit motive with most FOSS, at least not at its core, so there is little desire to data harvest generally. There is also a heavy overlap between FOSS advocates and privacy advocates, so they tend to be more privacy conscious via local data storage or encryption.
They could do it by not uploading any of the data, or if they do, uploading it encrypted with the only key being on the user’s device or a passcode.
Both are well established ways to secure data, but the company itself would not be able to interact with the data at all past storing it, so any features/revenue there would end.