Travel of low-income people also declined significantly while high-income travel did not. So quite literally the London congestion pricing got the poor off the zones.
From the report: "the highest income earners contributed to more of the revenue than the lowest income earners, making the scheme progressive in the scale of its equity impact"
It is quite likely that the lower income users are mostly retired people, and students, and they shouldn't be crowding the system at peak times unless willing to pay.
How would limiting access by adding a cost not obviously impact people differently based on their income? I'm struggling to see why this would even need to be verified by research.