I'm a little discouraged when I see articles like this that seem to be completely tuned for developers or look over completely decent pro-privacy alternatives like Apple. For example, the "best" calendar alternative is Etar which looks to a Github repo. Really? At the very least you could mention Apple Calendar. Is Maps.Me (which uses AdSense) really better than Apple Maps? I'm not a fan of hooktube either - it just…
It's not that "privacy-focused" tends to be conflated with FLOSS. Rather, it's nearly impossible to guarantee privacy in proprietary software. The transparency of FLOSS makes it trustless. Want to know what data of yours, if any, is being collected? Look at the code. This is why, when it comes to privacy, Apple isn't worth consideration. All we have is their word, and that simply isn't enough.
You can also use all sorts of runtime tools to see what a binary is doing at runtime, so I imagine it would be pretty easy to see if an application is phoning home, and where home is located, although the data is probably encrypted.
In fact, it might actually be easier for an end user to audit a binary using such automated tools instead of looking at the source code itself. At least with the automated tools, the tools can flag suspicious constructs in the binary that may indicate that it's up to no good, and do so in a way that is more understandable to the end user.