"I prefer" with a smiley and some mild snark isn't exactly a demand.

The context of other comments and posts by the same user caused me to read it as hostile passive-aggression.

Would it be worth having a convention at LW that measurements should be given in English and metric units?

Seems unnecessary. The general convention should be that people are entitled to employ the conventions and terminology in use where they live, or that they themselves are most familiar with. I wouldn't think of demanding that someone in another country talk about their purchasing habits in terms of US dollars, for example.