The Model T was released over a decade prior to the point mentioned in the article. Sure cars weren't the staple of American life they are today, but they were definitely accessible to the well-off people of the time.
They also weren’t as common - and so not as necessary.
Those who wanted to could get one but it wasn’t considered a necessity when there were so many other options - including sending a servant to the market for your servants, so no need to drive yourself.
The article mentioned that the Christies lived in a 4th-floor walkup in London.
When I first moved to the big city, I quickly discovered that the cost of parking and insuring my car was more than the cost of the car. This scenario was most likely worse back in those days as the infrastructure was designed to “store” horses, not cars.
Those who wanted to could get one but it wasn’t considered a necessity when there were so many other options - including sending a servant to the market for your servants, so no need to drive yourself.