Is this really so hard that it sets the bar higher to the point that it really works as a filter?

I know software developers who wouldn't be able to figure it out.

Sadly "yes". That said, it's not "so hard" in absolute terms, but that is precisely why it works so well as a zero'th order filter.

You wouldn't want to hire based on passing that, but you certainly want to NOT hire based on failing. (Depending on what you're hiring for, of course; I can't see not taking a sales guy to do sales if he couldn't do it.)

Actually if there business is cloud services, and APIs, then it might be a good idea NOT to hire a sales guy if they can't understand the basics.

That was my thought. I curl requests like this all the time. This is like a tire obstacle course, not a high jump bar.