While your hypothesis looks nice at first glance, it has a glaring hole: Experiments with turbine powered cars had ended before the first NOx legislation. The Chrysler Turbine Car dates from 1963. At the same time in the UK, Rover debuted the P6, which was engineered to host the Rover gas turbine, which was tested in a Rover P4 as the T2, T3 and T4 prototypes, but it never came to pass. Then you have some experiments with gas turbine powered race cars until 1968. The Motor Vehicle Air Pollution Control Act was enacted in 1965.
The experiments to put gas turbines into cars for sale ended two years before NOx mandates were enacted. They were continued for race cars, which aren't affected by the mandates, but fizzled out a few years later.