Being a walled garden is independent of privacy. The calendar app on macOS Calendar app allows me to share my calendars in an open format (ICAL) and interoperates with other calendar apps through that specification. It respects my privacy by not sharing anything I don't ask it to without being "walled".
Signal is open and secure; iMessage could be a non-proprietary format and remain just as private.
> all the press coverage of some app developer crying about App Store rejections or onerous rules.
As far as I'm aware a lot of these weren't for privacy matters [1] and are sometimes a little much [2] (this one is specially absurd: [3]).
[1]: https://techcrunch.com/2017/12/08/apples-widened-ban-on-temp...
[2]: https://www.theverge.com/2018/2/8/16992830/apple-emoji-crack...
[3]: https://medium.com/@alariccole/apple-literally-stole-my-thun...
PS:
> all the "open always wins" from the FOSS types
I don't think that means what you think it means. FOSS and privacy are tangential matters.
