"I rather sincerely doubt that there are 530,000 green vehicles a year going on the road."
According to the article, Georgia had monthly sales of 1400 before cutting their own tax credit. It would only take 32 states with sales like that to hit the 530k figure. Apparently Georgia is fairly big in terms of population and economy so it may be a bit of an outlier, but given there are 50 states in total it's still not an obviously silly figure.
However, that also highlights a rather big problem. Georgia had those sales before they cut the tax credit. After they cut it, sales dropped by more than an order of magnitude. Even if we take the figure given as a realistic one, you're not going to rake in billions by cutting tax credits on those sales because those sales won't be there if you cut the tax credit.