Though white dwarfs are dim, they are very hot and continue radiating heat for billions of years (a white dwarf can be seen in this Hubble Space Telescope image orbiting the bright winter star Sirius). That's long enough for life to evolve on a post-apocalypse world that survived the star's burnout. Where there's a thermal gradient from very hot to to very cold, there is always a chance for life to take advantage of it.
What's more, a long-lived super-civilization will eventually face the prospect of living around a white dwarf, as an alternative to abandoning the system and sending a small fraction of their population to colonize nearby stars. Coping with the dwarf may be more practical than assembling an interstellar wagon train.
After swelling to a red giant and convulsing off half of it mass as hot gas, the sun will burn out as a white dwarf in about 6 billion years. The remnant core of dense carbon "ash" will be no bigger than Earth. Glowing at tens of thousands of degrees Fahrenheit, the sun's surface will pour out ultraviolet light. But the sun will only be 1/1,000th its present brightness as an icy blue pinpoint star in the sky, even as seen from surviving planets.
The habitable zone will shrink to a radius of only one million miles, four times the distance between Earth and the moon. Therein lies the problem. A planet in a white dwarf's habitable zone will be so close to the dwarf that powerful tidal forces will heat the planet. This will evaporate away water the planet might have had, say the authors.
An analog can be found by looking at Jupiter's innermost Galilean satellite, Io (seen below). Though no bigger that our moon, Io has several active volcanoes spewing sulfur into space. Io's interior is kept warm enough to power the volcanoes largely by "tidal pumping" from Jupiter's gravitational grasp.