Polar bears eating dolphins for the first time as Arctic waters warm, page-28

That's my understanding too of the evolution of Polar Bears.

I think it is true that Polar Bears transitioned from warmer climate ancestors by evolving attributes to survive in harsh polar conditions .

But I don't think that this means that they are so tough that they can survive on a diet in which the availability of seals is reduced below a critical level . There is a need for a fat rich diet to enable them to survive in the harsh conditions and they have evolved the hunting skills to acquire this fat rich diet by being able to successfully hunt fat rich seals.

They also evolved physical attributes to make best use of this fat to survive - fat layers, fur to minimize heat loss etc.

I think that their evolution to to survive in the harsh conditions makes them dependent on fat rich seals even with their evolved ability to hunt other prey made possible by the seasonal variations in seal prey. Females, for example, need fat reserves that I believe can only be sourced from seals to survive the winter with babies.

Polar bear diet