You should read the materials you linked. They literally fabricated a new economic indicators index to provide a better score than the one previously used.
Please do quote me exactly where it says this new Detroit-only metric shows the city doing better than the metro-inclusive metric. I'm particularly curious to see where it says that since the article explicitly points out that the Detroit-only index performed worse than the metro-inclusive index.
> Moreover, the city’s economic growth more frequently dropped back into negative territory than the MSA’s: