My debit card is from Visa, and Visa is a supported payment system.
I think that's why he makes a point of saying American Express in the blog post too, it's one of the more obscure payment systems in the UK.
On a side note, I remember my mother, who runs a corner shop, giving anyone who pays on AmEx the evil eye because they have a higher merchant fee in the UK and it takes a few extra days to clear.
I wonder if Stripe UK have the same fees for AmEx...