I make it 8.9% with a binomial test[0]. I rounded that to 10%, because any more precision than that was not justified.

Specifically, the results from the blog post are impossible: with 200 samples, you can't possibly have the claimed 54.9/45.1 split of binary outcomes. Either they didn't actually make 200 tests but some other number, they didn't actually get the results they reported, or they did some kind of undocumented data munging like excluding all tied results. In any case, the uncertainty about the input data is larger than the uncertainty from the rounding.

[0] In R, binom.test(110, 200, 0.5, alternative="greater")