Simply put: you don't know. Whether it is who the lendee is, and whether he made a profit or not. That's kinda irrelevant.
The only thing that matters is that he's able to pay you back with interest.
That's why he has to put collateral (his margin funds).
Is it 100% safe ? Nope. But it is quite safe, I did it for a few months without any problem or loss.
But unless there's a -80% on all crypto at the same instant, you're quite safe.
Of course, you're still exposed to platform hacking (and the infamous bitfinex haircut style to recoup their losses).