It's also only 5% of the new cars registered in California alone, by your own data.
And that's why it's bad to compare things to cars. Framing is an argument. Comparing something to the equivalent car emissions frames the issue a certain way. By providing an absolute number of cars it makes it seem like it impacts emissions the same as a significant chunk of the car industry (it does not, it is, again by your count, 0.1% of the total).
The headline "Inhalers drive carbon emissions equivalent to 530,000 cars each year, study shows" reads very differently to "Inhalers drive carbon emissions equivalent to 0.1% of the cars sold each year, study shows", which in turn doesn't read the same as "Inhalers emit 2.5 million tons of CO2 each year". All of which don't cover the main takeaway from the study, which is that specifically metered dose inhalers are surprisingly pollutant and more research should be done on how to effectively replace them.
I can't tell you how easily they could switch to low emission inhalers, but I can tell you what a bad headline looks like, and this is one.