Seems unnecessarily cruel and divorced from reality, even if that is the point. In real software jobs, you don't have to learn a new language, framework, and especially a new paradigm in 50 minutes. Those are things that happen over weeks, months, and years. I don't know of any useful skill that adding even more stress to an interview will surface.

It's not going to do that. Over time it's just going to select front-end developers experienced with JavaScript.

An engineering mindset and systems thinking—or simple awareness of things like experimental design principals—are badly under-applied to software hiring processes, IMO. "What are we trying to select for with this section of the interview? Does this accomplish that, or does it do something else, or does it accomplish that but also do something else? Are we being any less humane and supportive in this section than necessary to evaluate what we have decided we need to evaluate? If not, how can we fix that?"

Your mileage will vary. I've been in enough interviews where it's a flawless-answer-or-fail.

What exactly would you be hoping to see the candidate do in such an interview? I’m moderately experienced on the front end (mostly jQuery and React, a bit of AngularJS) but if the task is making a SPA in vanilla JS I’d barely know where to begin. It’s pretty likely that I’d do hours of reading and researching before writing a single line of code. It would be one thing to talk about how you’d approach the task, but the GP specified their interview was apparently expecting workable code to be produced during the interview.