> Publishing reports like these without controlling for the economy/jobs/popularity of these cities seems to me to be an unfair attack on Uber/Lyft, where the real culprit might just be a strong economy.

I agree. That is why this report is so good. The section "How Do We Determine the Causes of Changes in Congestion?" covers this question.

" this time to systematically estimate the individual contributions to changes in roadway congestion of the factors of transportation network supply change, population change, employment change, and TNCs."

I agree that they have done a good job and that the conclusion that Uber/Lyft contributes to congestion increase is very valid and should be considered to take actions in the city.