metric system was the default on this site, and it is the US-imperial short ton (2000 pounds) which should be specified when "ton" is used.
I think there is some confusion here - we might be talking at cross-purposes.
SI units (or failing that, metric) is the preferred system, as per the FAQ you quoted.
That suggests, for example, that we should be sure to include metres instead of, or in addition to, yards.
However, it doesn't mean that we can, in the case of ambiguity, assume that the reader knows what we mean. So, it isn't safe to assume that an American reader (especially) will see the word "ton" and associate it with 1,000 kg.
But, in the case it is even worse, because "ton" isn't just ambiguous. It is (by only a small percentage) wrong in most areas.
In Australia, tonne and ton are different. Ton doesn't refer to 1,000 kg. I believe the UK (and most of the Commonwealth) is the same.
I understand in the USA, they prefer to use "metric ton" over "tonne", but "ton" still refers to the old-fashioned measure.
I have to emphasize that the error between the different measurements is small, and this precision was rendered irrelevant by the low precision of the original claim. I was being pedantic during copy-editing, and I was more worried about the lack of references than the error introduced by the ambiguity.