What you linked to is WASI-libc, which is a C standard library implementation on top of WASI - which itself is more low-level and less POSIX-like. https://github.com/WebAssembly/WASI
Here's the actual WASI spec:
https://github.com/WebAssembly/WASI/blob/main/phases/snapsho...
You'll find it matches the file I linked. Search for "POSIX" or "errno".
So I guess you could summarise it as: backward compatible-ish first, and then figuring out what a neater, more idiomatic world would look like second.
It's a pragmatic approach to bootstrapping the ecosystem, and there's no need for the POSIX-alike parts to hold anyone back, at least long term. Pretty soon you will be able to write apps that target WASI that end up reading and writing files in practice without the app even knowing what a "file" is (the runtime can just hand it an abstract IO object). That's a step beyond just not knowing about things like symlinks, and seems to me like a more "web-flavoured" future that can work even for CLI/desktop apps.