Doesn't make short variable names right. How do long variable names obscure code!! That's insane. There are many, many ways to obscure code, and there many ways to make it clear. Long-names is one of them.
Your "complaint" of go actually have nothing to do with Go itself...
In my opinion, a is an great variable name. Especially when it is used after calling a function like newApp(...). I may have picked a different name for newApp but it was still clear to me what it did.
Personally I think single character variable names are over used in Go as well, but at least they're generally restricted to the smallest of functions (eg struct methods), or instances where a longer name wouldn't be more descriptive (eg inside for loops (for i := 0), or b[], byte arrays.).
However in this specific example, I think "a := NewApp(server)" is both readable and overly terse. So I do find myself agreeing with the OP in their example.
I've looked at a lot of other people's Go code and I have had no issue with 1 character variable names, but maybe that is just me. The methods/functions is generally small enough that it is obvious to me what is happening.
I have two metrics for variable names. First, is it clear what the variable is? You say yes. Second, is it not obnoxious to type? Clearly not.
So, at least by my metrics, it's a great variable name.