Yes, it is a refundable tax credit. As you pointed out, a "refundable credit" means you can receive it if your TAX is less than that amount. But the calculation of the credit is limited to the lower of $2553 (in your case) or your actual cost.
So if your tax return only shows $5000 in tax, and your Premium Tax Credit is $24,000 (if you did not receive any advance credit, but choose to take it all on your tax return), you could still receive a refund of $19,000 (it is not limited to $5000). But that does not change that the credit itself is limited to the the amount of the cost of insurance.
