This would logically be read as "bottom 30% of Americans, ranked by car ownership [rather than net worth or income]". I suspect that there are in fact more than 30% of Americans who own zero cars.
18% of Americans are 14 years old or younger [and presumably almost none of them own a car], so if 15% of those 15 years old or older don't own a car, the total is over 30%.
Fair point. Even solving for age, I would assume this holds as cities with metros mean roughly 80% of people don't even need a car.
It's still an absurd comparison, as 0% of people don't need money.
That this somehow reads to you as totally normal and not at all anything to worry about tells me a lot about your perspective.
edit (addendum):
> "Joe Biden owns more cars (3) than the bottom 30% of the US SUMMED (0)"
Fairly certain the bottom 30% owns more than 0 cars in total.