I suspect it was whitelisted. But it would be interesting to test the hypothesis, say by ordering a product or service from a different US merchant using the first line of your own foreign address and the US merchant's city, state, and zip for the rest of the details. I suspect it wouldn't work, and the transaction would be declined. Worst case scenario, your credit card company might suspect a fishy transaction and freeze your card, so best tested with a non-essential credit card. (And for a product, you'd still need to enter your foreign address for delivery and that in it's own right might likely kill the transaction.)
Had the TaxAct trick not worked, I would have next tried using a virtual pre-paid card offered by US Unlocked which provides you a US billing address. (And for products, you can use one of the shipping address of one of their US re-packagers who forward it on to you.)
For TaxAct though, I was pleased that the trick worked because I wasn't looking forward to trying to use a more complicated (and more costly) system like US Unlocked.