I agree with this statement if you interpret "SB 375 implementation" to refer only to the precise achievement of CARB's per capita GHG reduction targets. If the model is not accurate and valid, it will fail to predict the effect of transportation policy and investment changes on travel behavior and thus GHG emissions.
But is the purpose of SB 375 to truly reshape our urban form, then the primary barrier is not our modeling tools, but rather the political will to do everything we can to improve transportation and housing choices for all California residents. We know the policies that will move us in the right direction - the best transportation policy is intuitive. HOV lane conversions, dropping transit headways on high-volume routes, jobs-housing fit (especially to accommodate low-income folks that typically in-commute, potentially providing relief from hellishly long and complex transit trips), selectively eliminating egregious highway bottlenecks, probably not constructing entirely new highways or interchanges, promoting active transportation by mixing land uses and income levels with greenspace, promoting user fees over sales taxes for finance, etc. In California, we've been aware of all of these methods (basically) since the late 60s.
The more fundamental question - the real problem with SB 375 - is whether unelected regional governments composed entirely of local electeds are capable of implementing a statewide policy goal. California's transportation planning history indicates that we are loathe to enforce state-level goals on lower units of government. In fact, our laws have evolved to make this nearly impossible. SB 375 creates a state-level mandate but does not touch the system of transportation and land use governance. How can we expect mid-level units of government with no enforcement authority to reshape cities now or ever?
I've been into California's planning history recently. It's got some surprisingly timely insights for today's debates. In May, 1977 the Director of Caltrans, Adriana Gianturco said,*
I do not have a grand plan.** I just do not think it is possible any more to come up with grand plans that last for 20 years. Conditions are just changing too rapidly. It is unrealistic to think that way. I think we need marginal changes, fiddling with the system in relatively minor ways, seeing whether it works, proceeding more along a certain line if it works, dropping it if it does not work.
One of the reasons why it's appealing to not enact change in this manner is that the long-term plans we enact (RTPs are typically 30 year plans coupled with shorter term improvement programs) normally don't have to demonstrate conformity with any performance targets. This is what's changing with SB 375, but it's not clear why such a long-term target is necessary. Why couldn't we use best practices combined with periodic updates? Let's still use models to plan particular infrastructure decisions, but why rely on them to meet climate goals? Looks like we need some wisdom from the 70s to be injected into current planning practice.
* Quote is from the May, 1977 issue of Mass Transit magazine.
**Under AB 69 (Chapter 1253, Statutes of 1972) Caltrans had been given authority to conduct (potentially meaningful) statewide planning. After fits and starts they still (kind of) enjoy this authority, but the statewide plans don't seem to really mesh well with regional plans anymore.