I believe their point is that, because the US is generally regarded as 'to the right of' many countries in Europe, for example, a slight rightwing bias looks to us (I'm in the UK) like an extreme rightwing bias. Heck, even a completely neutral stance, as far as the US is concerned, will look like it veers to the right for us. Note that I'm not saying anything about how I believe the twitter algorithm should work, this is just my interpretation of what that commenter meant.
Europe is only a tiny fraction of the world, though.
Compared to India, Japan, Singapore and (depending on the metric) China, the US looks clearly left of center. Compared to most of the Middle East, the US looks extremely left-wing.
