> I'm surprised the author forgot to consider platform architectures

That is mainly for binary distribution. Most C++ package managers currently are source-based because binary compatibility is not a easy problem(there are some package managers trying to solve it though).

> Unfortunately, it is still terribly easy to write non-portable code with C++

Yes, it easy to call into a platform-specific API, so cross-platform portability is limited by the testing resources for the package maintainer or library author.