n+n+k vs k =checkmate.

why are two knights insugnifican

because theres no way to force the mate.. the move prior there will be a choice for the lone king to go to the corner, or another square. as long as he does elsewhere its safe

The position is impossible.

White's last move was obviously Ng6#. How did the Black King get to h8 on the move prior?

Below is a legal position with 2Ns mating the Black King.

Like this. Of course, Jarkov is correct that the mate can't be forced, but the position is possible.

Not true. Black just made the poor choice of Kh8 instead of Kf8. Checkmate isn't forced the strong side can still mate if the weak side doesn't pay attention. 

I see, I see, my reasoning was flawed, (of course, the King was moving out of check... I somehow got it in my head that the King can't be coming from an attacked square... absurd!... You must forgive me I've been arguing with the wife all evening and it's obviously scrambled my brain!)  you all are quite correct!

...Kh8?? I think noone will play like that. That mate is logically imposibble.