@Jo *********
never trust an embassy website for rules about immigration policies within Thailand. Two separate government departments and often they get it wrong.
To be fair I believe that the actual law states 10,000 baht per person or 20,000 per family (although when I tried to find a reference to this I couldn’t), however if asked to show cash it will need to be 20,000 baht or equivalent (40k for a family).
Another possibility for the confusion could be that people entering under the VOA scheme (not visa exempt) are asked to show cash of 10,000 per person or 20,000 per family.