The parent postulates that a "Nakamoto" type 2 civilization will convert all the energy of its star into heat doing bitcoin calculations. Such a star would disappear from visual light (Dyson sphere stuff). The Clarke story is about a religion that uses a computer to calculate all the names of god, bringing about the end of the universe. The story ends with stars disappearing. Both have big computers doing math that results in stars disappearing from the sky.

The joke is that rather than calculating the names of god, maybe we will end the universe by finally getting to the end of bitcoin calculations.

You need to read the short story referenced above to understand the context.