Translation:That one's motorcycle turned to the side.
sin refers back to the subject of the sentence, which is motorciklo, not the referent of ties, ĉu ne? So sin unambiguously translates as it in English. The motorcycle turned to the side, not the rider.
However, I am not sure what turned to the side is supposed to mean. Turned to the right? Turned to the left? Pulled to the side of the road? Fell on its side? It feels like either the verb or preposition is wrong in English, even if the Esperanto actually makes sense.
I really struggle with what they are trying to convey with the Esperanto version of this sentence.
According to the Lernu dictionary: turni - to turn, to turn around (turni sin)
So did their motorcycle turn around? What does flanke add to the meaning? Is there another way to turn around? Doesn't turning already indicate to the side?
There are some clearer definitions in PIV. One in particular applies here: ‘Alpreni novan direkton’. Example sentences include: “tie la karavano turnis sin al la sudo”, “(…) ĉu la ventoflago sin turnas”. So the motorcycles simply changes direction; it turns. The flanken-part indicates where it turns to. In this case sideways, to the side. As you notice in the example sentences, the turning part need not be to the side, but can be any other direction than the current one.