If the candidate asks if you're sure you want them to use any language and you say "yes", and then get pissy when they do, the candidate isn't the one who sabotaged anything and they're dodging a bullet if they "fail".

Interviews go both ways ... I don't think they lost out on anything they wanted.

Use the right tool for the job. Thats engineering.

Instead you insist we should solve a nieche problem with a ill suited tool, while inventing a costume solution when a standard solution exist.

They dodged a bullet. It would have been hell working there.

Why would you ever want to work somewhere that clearly employs such unqualified individuals? And not only that, but allows those individuals to be the face of their company to prospective hires?

A company's interview process tells you a lot about how the company thinks and operates. This was was surely a dumpster fire.

What's the point of doing well if you already determined you wouldn't even look at their offer?

Sabotaging? The candidate learned that their interviewers, and probably the company as a whole, isn't curious about languages or stuff that is outside of their wheelhouse.

What if the interviewers decided to ask the candidate about their language choice and trade-offs between different languages? Wouldn't that actually give them more signals into the skill of the engineer, rather than just blindly following their script?