In fact its the dialect spoken by the majority of people in Croatia (even though Stokavian is the "official" Croatian) and sometimes argued to be more than just a dialect. https://en.wikipedia.org/wiki/Kajkavian
Personally I always found the comparison "Croatian vs. Serbian is the same as US English vs. British" a great oversimplification. It's not just an intonation or alphabet, it's also a lot more different words (that might mean the same things) than for example between the variants of English. Yes, people understand each other - but they're also attuned to all the different dialects within the "main language groups" so they actually know/learn more words for same objects (for example Croatians will be happy to list different words for bread/tomatos/etc i.e. synonyms that they might know the meaning of but don't necessarily use daily - and those different synonyms depending on the dialect might be closer to Serbian, Croatian, Slovenian, Italian, Turkish, Hungarian or old Austrian).
In a way I think this invalidates the premise of the article - that there is "a peculiar form of theater going on" and that the governments are pretending by needing court translators. If the countries are using not only different alphabets, but in part also different words (even though that other surrounding countries might somewhat understand those different words) - whats a viable alternative then?