If I was 90% sure that humanity is facing extinction as a result of badly done AI but my confidence that averting the risk is possible was only .1% while I estimated another existential risk to kill off humanity with a 5% probability and my confidence in averting it was 1%, shouldn't I concentrate on the less probable but solvable risk?

I don't think so - assuming we are trying to maximise p(save all humans).

It appears that at least one of us is making a math mistake.