Shees, can't we have any fun before someone rains on our parade...
The art in the twitter post is obviously inspired by a parade, Mardi Grass or the Rio carnival. Those are all inspired by far older European events/parades that were eventually exported with the colonization of the 'new world' and adapted. Just like Christmas now isn't Christmas from a millennium ago. But we've seen Santa Claus show up quite often in D&D (related) adventures. It's what we know now, it's what's culturally relevant to the player base. Very few would recognize 'Masopust' especially in the US, where WotC is located and most of it's target audience is located. WotC is nothing but US-centric.
The Radiant Citadel is it's own separate 'world', so it isn't strange that it has it's own technology, just like the Modrons have in Mechanus, the Spelljammer universe, Eberron, or all the different realms in Ravenloft. A fully medieval metal plate armoured warrior has no place in Dark Sun, etc.
That said, I don't like the illustration, my problem is the face in it. I really like the illustration of the old woman that's shown later in the thread, and I do not like the shoes for my regular D&D campaign. But would certainly use it for something more modern like Vampire or Shadowrun. Or just cut off the underside of the illustration in a D&D campaign. The white high heel shoe in the other illustration isn't as present/focused as the other illustration.
The whole Radiant Citadel book wasn't aimed at me in the first place and I disliked it's presentation, wholly due to the WotC marketing team focusing solely on "Look at how culturally sensitive we can be by employing these people of diverse cultural backgrounds!". By the time the adventure actually launched I had no idea what the adventure (collection) was all about. Three years later I think that there are a lot of good ideas in there and quite a bit of good illustrations. I have zero interest in running an adventure in essentially 19th century southern US, just like I don't want to run a D&D campaign in 1940s Poland... Some of those settings deserve imho their own campaign/world book! Which never happened, it was just lip service. But maybe something is better then nothing...
Not everything produced is aimed at every D&D fan/group, The Radiant Citadel was one of those products.