Doesn't really help in this context. What's the difference between "pythonic" and "idiomatic"?
Does Coconut being pythonic mean it's written in idiomatic Python (then why is it a different language?) or does it mean it's written in idiomatic Coconut?
They just mean "this language looks in general like Python." You're overthinking it.
It's like saying that JavaScript looks C-ish or Java-ish. It definitely doesn't look exactly the same; if you see e.g.
var result = [].slice.apply(vals, 0);
then you can be highly certain that that's JS and not Java or C. But the idea of having blocks of code enclosed in curly braces, which are formed out of statements usually delimited by semicolons (except for special forms like if-statements and for-loops which don't need to be followed with a terminal semicolon), etc. is very much a C-style thing.
Pythonic is just short for idiomatic Python. It's a different language presumably because it introduces some new language constructs that are not compatible with Python. Perhaps they're saying it's 'Pythonic' because of some loosely defined notion that the new constructs feel and behave in the same way that Python does, but that's just speculation.
Does Coconut being pythonic mean it's written in idiomatic Python (then why is it a different language?) or does it mean it's written in idiomatic Coconut?
I don't think they thought this through.