Hello there, I would say no to the question of whether APFS is a POSIX-compliant file system like Btrfs or ZFS. Actually, I'd rather say that macOS is POSIX compliant.
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems.
POSIX defines both the system-level and user-level application programming interfaces (API), along with command line shells and utility interfaces, for software compatibility (portability) with variants of Unix and other operating systems. POSIX is also a trademark of the IEEE. POSIX is intended to be used by both application and system developers.
So you can draw the conclusion that the portable operating system interface is more like an operating system than a file system.
In our article macOS vs Linux: the Battle of the Best, we refer to that macOS is developed by Steve Jobs when he was devoted to NeXT Computer company, and he brought this famous operating system to Apple Computer later, where he hit the ball and got big success thanks to the macOS. In terms of technology, macOS is built on the basis of March Kernel and UNIX derived-BSD.
The BSD Unix Kernal is known as Darwin and is open source, which is accessible to all. While other parts, for example, GUI Partition Map and core applications, are secure and close-source. And that's why the macOS has a combination of Unix technologies and Apple's proprietary technologies.
As a result, macOS is a POSIX-compliant operating system, while APFS is not.