Two months ago my partner recorded me speaking in my sleep. I was speaking fluent Mandarin. I always thought sleep time is used for learning (among healing etc), but now I am convinced.

Well you’ll have to give us more. Do you speak Mandarin at all?

spoiler, he is Chinese and only speaks Mandarin

But writes in English. Very rare.

I've met a few people who write English but can't speak it. One of them is polish and learned to type English to play online video games.

And, what was the partner's ability to benchmark? What is their level of familiarity with the language?

I would love to believe.

It was a recording. I dare you to ask for it.

I shall not be ensnared by your schoolyard dare. You cannot manipulate me so easily. OP, however...