They should have perhaps included another variable in their study.
It is their right to decide with which methodology they will conduct the research, as well as in which countries they will conduct it, so it is possible that some countries will not be on the list at all because they have no data for them.
I have bought at BTCATMs in parts of Europe. Switzerland and Austria have very friendly BTCATMs, in that there is no KYC for smaller amounts (250 euro for Austria, IIRC), and there are lots of the machines in those two countries. I have not been to Poland, but coinatmradar.com shows a lot of machines there as well.
This is just proof that countries within the EU implement laws differently when it comes to KYC and AMLD. It is obvious that the degree of development of society and corruption and crime rates play a role in such decisions.
Germany, on the other hand, has a very low number of BTCATMs. The UK used to have a lot, but their regulations have strangled that.
And this is a country where you pay 0% tax if you sell crypto 1 year after purchase, so it would be logical that people are more interested in such an investment. The UK is an indicator that everything can change literally overnight, and this can happen in any country when the authorities change the rules of the game.