I didn't adjust the $9000 for inflation because according to the CPI calculations there's been ~0% inflation for cars. So if they're right a new 1990 Mustang would be priced at $9000 today in today's dollars.
Meanwhile, that $20k buys at loss less food, healthcare, education, house, etc which is why "today's dollars" are worth less and why it's fair to say cars got cheaper in inflation adjusted terms.
Yep this was my point. We can see if the CPI is being calculated correctly by comparing cars which had the same price in each year and see if they really are about as good as each other. Whether you want to call this 'inflation for cars' is just semantics.
(The original article says a Mustang in 1990 cost $9000, but I've also found sources for $10000 and $20000 so you might be right on that point.)