Does that mean you can run Irix binaries on MIPS Linux?

This dates back to how Linus did the first non-x86 port of Linux to the DEC Alpha. Rather than copying the x86 linux syscall conventions, he used the DEC OSF/1 syscall conventions to ease bootstrapping. This is a perfectly sane approach. But he probably should have reverted to a native syscall numbering rather than leaving this bootstrapping hack in place.

Different platforms have different sets of registers and alignment requirements. You want to pass things in registers if possible. If you want to avoid inefficiency, you need platform differences. And this is all hidden from users and even developers anyway.