The type is library code, the only part that's a language feature is the grammar. ValueTuple is a type like any other, and the CLR will need it referenced to use it.
Microsoft seriously needs to rediscover the value of simplicity. I don't need to install some third party library to use tuples in python. Tuples being part of the language means it just works.
My point is that it's not even an assembly which is part of the installed .net framework. It's an assembly that needs to be downloaded from the internet. That's not exactly the syntax of the language nor a model of simplicity... Again compare it to python.
I pretty much agree with the essence of what you're saying. However, it could still be syntax. For instance linq query expressions are a defined to translate into various .Select(), .Where(), .SelectMany(), and other method calls. However, those methods aren't necessarily defined in the standard library. I mean, they usually are, but they could be just as easily defined for a given type in a 3rd party library. So this isn't an entirely new concept in .net.