But not comparable environments. Most IOT environments are very small parts of very big systems, and POSIX defines a system with a teletype and a line editor.
I seriously doubt the value of the existing codebase. Saying code made for a server (like most existing unix code!) is fine for IOT feels wrong, and not just because IOT devices have kilobytes of memory and servers can have gigabytes/terabytes.
I don't think that a universal OS can work well when we know that universal programming languages, data transfer protocols, and everything else didn't. Imagine if we were still doing everything in PL/I. We recognize now that different programming tasks need different programming environments, but we still don't think that different programs need different program environments. It's just strange to me.