This is a qualitative, not quantitative study. The goal of qualitative research (in cases like this) is, essentially, finding categories, not the relative distribution of the population in those categories. I've seen well-executed qualitative studies with less than 10 participants, 53 is actually on the higher end for this approach.

You usually try to select for a pretty diverse cohort (which the authors did in this case), not a representative one.

A diverse cohort means you'll probably have all categories that matter. A representative cohort means you'll have a distribution over these categories that is pretty similar to the real world.

The result here is essentially displayed in Figure 1. One could now follow up with a quantitative design to e.g. discern which particular step is the most common obstacle.