I'd call it a fundamental semantics issue. That discussion is mostly framed in terms of the compile-time problems that arise, but that brief aside about the reflection package is deceptively short, because it means that the entire compile time set of problems is also lifted into runtime, where the problem is even worse. (To translate for someone who isn't a Go expert, the reflection package has enough power that it can dynamically instantiate new types that the compiler did not and can not even in principle have seen at compile time, which may then require new methods to be created. The current Go runtime has no run-time code creation capabilities. [1]) Without the reflection problem, it's annoying but at least solvable in principle, but I don't see any reasonable design (without a fundamental overhaul to specify a runtime-available compiler in the language spec itself, which is a huge step) that solves the reflection issue.
[1] Which personally I consider almost a feature. YMMV. Runtime compilation is very cool in a lot of ways but is a hell of a subsystem to try to secure, just by its very nature. There's a lot to be said for simply not having that capability unless it is integral to some particular program.