They didn't solve it by 90%, they lied with statistics, and compared exponential-growth forecasted counts with point-in time counts.
The reduction was closer to 60%, and in the past few years, they've stopped investing in the program. It's been a good thing, but without ongoing effort and investment it will regress back to the mean.
In ten years, incidence of chronic homelessness in Utah went from 1,932 to 178.
Solving 91% of homelessness is a fantastic achievement, even if the remaining 9% needs further work.