Just quoting the part of the article that you are referring to:
This is the only relevant measure when looking at the specific issue of deaths in custody.Of those arrested, 79% were white and 85% of those who died in custody were white.
Meanwhile, 9% of people arrested were black (which is disproportionally high) and 8% of those who died in custody were black.
So, over the last 10 years, a white individual who has been arrested was about 25% more likely to die in custody than a black individual who had been arrested.