AWS supports billing in multiple currencies: https://repost.aws/knowledge-center/supported-aws-currencies

So how is it a "foot gun" to add XTS as an additional currency, for internal use only?

I presume AWS sets its prices in USD, and then converts them to the other currencies using the relevant exchange rate - the services themselves don't know about UAE dirham, but the billing system does. So XTS just becomes another exchange rate. You could even fix it as 1 XTS = 1 USD, although choosing a different exchange rate than parity is likely to surface more bugs.