Yes, you get 6x points for online spending with the SCB Amex. Have been using it online for many years.
Most recent purchase - over US$20,000 (and only US$700 of it mine) on a new but well set up online shop selling high-end men's stuff (John Lobb, McQueen, YSL, etc.) - they had a VIP sale, lots of stuff 50% off. The bill was in British Pounds..... a very easy way to earn over 60,000 miles.
Another example - buying from a well-known French leathergoods/trunk maker - sent directly to Hong Kong (instead of paying 30% extra at their boutique in HK) - purchased 19 of the same handbag (in different colors) and a briefcase for me. Don't get me wrong - I'm not re-selling the bags - I just gathered a big group of friends who were willing to split the hefty delivery charge. Another 60,000 miles earned without any effort.
Oops, I've said too much.
The answer to your question - yes, SCB Amex doesn't care how you spend it, online or not - as long as it's in a foreign currency.