I think their aim was to highlight the distinction between a language and an implementation of that language and possibly argue that only a particular implementation should be referred to as compiled/interpreted.

What I understand is that they invented a language called Crystal. And they made a compiler for it. The phrase "compiled language" is not very meaningful. It is quite possible for someone to make a Crystal interpreter.

Sure, but the goal of the language creator is to create a compiled Ruby-like language. As such, Crystal is by design meant to be compiled. Which is also the case for languages like C, C++, Go, etc.

What properties of C indicate that it is meant to be compiled? The word "compiler" does not appear in the ISO/IEC 9899:1999 standard (aka C99) in any meaningful way (there is just one sentence in a footnote).

That it's fairly close to assembly for a high level language, and most C that has ever been written has been compiled.

Okay, but it still seems strange to claim compiled-ness as some sort of intrinsic property of a language.

It's more a general descriptive of languages designed with compilation in mind.