Also, looking at the price for the most expensive one (~50USD) makes me think: did they maybe just took the price of an unlimited plan and used that as price for a GB?
Lastly, average in this analysis does not really make sense to me. Especially as some very expensive plans can skew it to very large values.
Not sure where you get that impression, but I don't think it is true at all. The following are the standard CH mobile plans by the largest providers in Switzerland.
Swisscom: 3×unlimited, 3×limited
Salt: 3×unlimited, 2×limited
Sunrise: 1×unlimited, 2×limited
In total they have 7 unlimited and 7 limited plans. The unlimited are mostly very expensive. Also note that the article mentioned they measured 14 plans in total which exactly matches the total number of plans above. But that of course might be a coincidence. Most plans of smaller providers are also limited in my experience.
Still I do not understand how would they have extrapolated price per GB when considering an unlimited plan. Can that be that they just took the price of the unlimited plan and assumed that's the price of 1GB? That would not be wrong, but a bit deceiving to say the least. Otherwise I would not know how the max price is ~50USD. Swisscom gives me ~15CHF per 1GB, and that is usually seen as the most expensive company.
Right now for example I'm paying 50$ for 30mbps unlimited* yearly plan here in Thailand. The map in the article puts Thailand at avg 1.2$/GB which would net 42GB a year for 50$ which is a much worse deal.
Finally I feel it's very hard to measure this. If you look at the article's map the avg/cheapest/most expensive vary by an absurd margin. Again Thailand is 0.2$ cheapest and 7.2$ most expensive — that 32x difference. This data seems to be absolutely useless for drawing any sort of conclusions.
\* unlimited up to some point.
I believe the report is written in the UK.