I'd argue that Rust is as portable as C++, if not more so, in the sense that there are no real barriers in the language itself that limit portability. It's harder to write un-portable code in Rust than in C++, because a lot less in idiomatic rust is undefined or food-gunable.

Of course when it comes to practical portability today then you're right as there are plenty more C++ compilers for different platforms than Rust compilers (then again, there are more C compilers than C++ in that regard). This may eventually change over time, tho, but it sure is something to consider today.

PS and edit: I don't mind you disagreeing with me, but would anybody care to illuminate me as to what your disagreement is about?