The $1 billion is not a forecast as much as it is a mandate set by the legislature. The goal is to hit $1 billion dollars whatever it takes.

In general these types of programs never generate the revenue that is expected. Throw in all of the carve outs and the 'ingenuity' of people to find workarounds and you'll be hard pressed to hit that number. As it's a mandate to hit $1 billion the only option at that point will be to raise the fees to make up the the carve outs and work arounds. Of course this will not prove to be popular and will likely get a lot of resistance. In fact this study determined the most numerous population segment of commuters are police (which likely won't be subject to the congestion fees).

"The tract will also surely be the epicenter of the fight over congestion pricing exemptions, since any exemptions given out to people driving and parking in this slice of lower Manhattan could eviscerate the entire program."

https://nyc.streetsblog.org/2022/11/04/congestion-pricing-study-finds-law-enforcement-are-manhattans-most-numerous-car-commuters/

However this not the important point. The main problem is that this new revenue will not fill the hole. Even if it was $2.5 billion the MTA would find a way to say they are at shortfall once again.