Both the popular article linked in the op as well as the actual paper seem to use the terms “liberal/conservative” and “leftist/rightist” interchangeably. Quote from the paper:
It is necessary to note that, first, similar to previous studies on this topic that consider the left–right dimension equivalent to the liberal–conservative dimension (Fuchs and Klingemann, 1990; Hasson et al., 2018), throughout this paper, the terms leftist and liberal (and similarly, rightist and conservative) were used interchangeably. The liberal–conservative dimension is often used in the United States, whereas the left–right dimension is commonly used in Europe and Israel (Hasson et al., 2018).
There were “only” 55 participants, but I assume that if some of them identified as socialist, they would already be included under “leftist/liberal” for the purpose of the study.
Germany for example adds up to 101% though, that would be impossible if they had truncated. Most likely they simply did round to full percent.