I think it makes sense in their head that there exists a dichotomy between left and right (restricted to liberal/conservative in a very US-centric way) and that they are on the left of that spectrum. Then, being "as left as can be" is a matter of being more what they consider left, which, indeed, they are.
God, I forgot how annoying I find this phrasing they adopt, especially when you know they haven't done the least bit of reading to inform their opinions.
Ah, yes, of course. I specified western because I had in mind certain people I've seen using lemmy, as I mentioned in the OP, which, as far as I've gathered, is populated mostly by westerners.