The letter talks about a man standing on a burning platform choosing to jump into the icy waters instead.
In this metaphor, Nokias actions were neither staying on the platform not jumping into the water but shooting itself in the head instead.
If the internally Nokia OSes were not going anywhere, which looked to be the case from the outside, the obvious solution was to go with Android. Something that already had the user base, had the apps, had the platform, was open source so it allowed for innovation and differentiation, and Nokia could tailor it to work well with their hardware.
Instead, he chose to go with Windows Phone which had absolutely no benefits. And had severe restriction the kind of phones Nokia could create and on the modifications they could make (they could barely even reskin it) and did not allow Nokia to leverage any software or hardware prowess they may have had.