And one can invent those libraries for another language and can match Arc in the challenge using those invented libraries -- at least you don't seem to be denying this.
Depends on the language, obviously. It seems unlikely you could in C, for example. Presumably the problems you'd encounter would gradually decrease as the language grew more powerful. That's why I phrased the problem as a challenge. I was curious to see what happened when you tried to solve this very simple problem using existing language/library options.
You've widened the scope of "language" to its libraries. And that's entirely fair. So, yes, Arc has it "built in", but Arc itself is (IMO) only marginally more mature than the libraries one might invent for (e.g.) Haskell to do the same thing (ok, I'm being a little unfair here, but not that much IMO). And one can invent those libraries for another language and can match Arc in the challenge using those invented libraries -- at least you don't seem to be denying this.
So, then, what's the point? That Arc already has the libraries available? That the Arc libraries meet the challenge? It certainly isn't that those libraries aren't possible in another language. The challenge means (almost) nothing with regards to comparing programming languages as you first implied, and is more about what tools and libraries were invented along with Arc to develop web apps.