That’s not what the phrase implies. If you have a C program from 1982, you can still compile it on a 1982 operating system and toolchain and it’ll work just as before.

But if you tried to compile it on today’s libc, making today’s syscalls… good luck with that.

Software “rots” in the sense that it has to be updated to run on today’s systems. They’re a moving target. You can still run HyperCard on an emulator, but good luck running it unmodded on a Mac you buy today.