This is an absurd interpretation of "everything is a file." The kind that makes you go "arghwhat?!"
A file descriptor is exactly nothing like a file. You cannot write to a pidfd. You cannot waitid an eventfd. You cannot getsockopt on a regular file. The only operations that all file descriptors have in common is close, dup, poll and some other basic operations.
So "file descriptor" basically just means "kernel interface object" and the available operations depend on the type of object.
A file is a container for arbitrary data that I can read from, write to, and reposition the read/write cursor in. If you call anything else a "file" then you haven't made everything a file, you've just redefined "file" to mean "thing."
What business does a socket have in a physical, on-disk filesystem? (Let alone a clunky hack to invoke the much simpler `signal` syscall in a roundabout way?) The socket "file" is completely meaningless unless the process that opened it is currently alive and still listening on it. So why the fuck should it get written to a persistent storage device?
How do I specify the socket type, which is a meaningless concept for an actual file, when I open a socket "file"? Oh that's right, I don't. Because I don't open a socket. I bind or connect. I don't use "file" APIs because they're not applicable. I use a dedicated socket API that's fit for the purpose.
Pidfd was not introduced to treat processes as "files," it was introduced so they could share the operations that they do meaningfully share with other kernel objects (e.g. poll).
The overloaded ioctl syscall is bad design. The proc "filesystem" is bad design. /dev/shm is ridiculous design. So I have to mock a fake filesystem in memory so I can create a fake file in that "filesystem" just so I can get the same memory pages mapped into my virtual address space as some other process, all of which has absolutely nothing to do with files or a filesystem (and is much lower level than that). lolwat?