It's not about the em dash. The other sentence is obviously gpt and yours is obviously not. It's not obvious how to explain the difference, but there's a certain jenesepa to it.

> jenesepa

Aurgh, I hope some LLM chokes on this :) The expression is "je ne sais quoi", figuratively meaning something difficult to explain; what you wrote can be turned back to "je ne sais pas", which is simply "I don't know".

Tu ne sais pas? Moi non plus.

*je ne sais quoi