The sun imparts a certain amount of energy into the earth through tidal forces. We can either capture that energy for our use, or let it heat the Earth (https://en.wikipedia.org/wiki/Tidal_heating). The slowdown of the Earth's rotation is the same either way. The paper demonstrates that if you could capture an absurd amount of energy from tides, the earth's rotation would have to slow. But this is vacuously true: you simply can't capture that much energy from tides.