Job interview in target language as a curveball

I just had a final round job interview for a process engineer position. I was told to prepare for very specific technical questions (explaining P&ID diagrams), which they didn't ask about. But in the end, they brought in a TL native speaker and he just wanted to talk about technical aspects.

I speak the language pretty well actually (tested at C1). But I hadn't used it for real in a good while, only consumed books and movies, never needed to talk about very specific components of biogas plants before.

It went relatively well given the circumstances, but I can't help but feel disappointed in myself or like an imposter. I'm usually a lot more fluent and today I couldn't think of some words, but mostly technical ones. I had half-heartedly prepared some things in TL but I mostly looked at the very technical details and wasn't focusing on the language. It was listed as a plus and I was confident I could answer the more usual questions. And I can explain more technical details at home a lot more fluently as well!

He did give me positive feedback and I think they'll give me the job based on some other comments. If I don't get it, I can't help but blame it on the TL aspect. I can do everything flawlessly in two languages already where there's literally no difference to me if one or the other language is spoken. TL is my third language.

What are interviewers looking for in this situation? The native speaker gave positive feedback, said I just needed to revise some technical/chemical vocab (I couldn't think of two words and explained them, which wouldn't be so bad but it took a while for me to get there and I stumbled across some other words as well). I said that I hadn't actively used the language in three years, just consumed it passively, and I told them if they give me a week to get more familiar with the technical terms I'd be there.