Ich denke, Partys sind gut zum Deutsch lernen.
This seems actually a very subtle mistake. You can search the term "zum Deutsch lernen" and you find lot of examples using this writing. Even on sites dedicated to learning German.
So it's either correct, or nobody notices the mistake.
You would indeed write: Ich bin hier zum Arbeiten/ Lernen. (Many natives get that wrong and write: Ich bin hier zum lernen.)
Searching the term, I came across: Ich bin hier zum Deutschlernen.
Here it's a combined noun. (Writing looks correct to me. Kind of strange, but coherent.)
There is also the form : Ich bin hier zum Deutsch Lernen.
This looks gramatically correct, but kind of odd (I am a native German speaker).
In many languages you have a difference between written and spoken language, and maybe the author just didn't worry too much.