To add: Choosing which risks to tackle should definitely be weighted by difficulty, impact and available resources. Also, this feels like it's forcing an external portfolio manager view on the internal operations of a startup. Nothing wrong with this - it's just odd to me.
My interpretation of the presented material that no, difficulty doesn't matter, "available resources" too, "impact" - well, the impact you should strive to is exactly the reduction of the most critical risks. Using the example from the linked material: surely if you can't swim, learning may be quite difficult.
Impossible to prioritize which risks to address without considering difficulty and resources.