Fair enough, that was too strong a claim. But it is more portable than an extension module compiled with the system compiler.

It's not portable enough to let me take IR generated for my x86 laptop and run it on my arm board, in general, even though both platforms are ILP32, little-endian, and running the same OS.

Yes, but this is a problem with the source language and/or the host system libraries, not with LLVM IR itself. There is a broad domain of applications for which it is portable.