This for example, clearly communicates an emotion and a space. It doesn't sound what you expect of "medieval fantasy" (I.E an imitation of Romantic music or sometimes Baroque music). Yet there is clearly a theme there.
I'm only half-joking but if I make all these concessions, why not substitute all of Vermintide's music for Quake II soundtrack? The genre is the same anyway
In both cases it is a baggage you associate, a preference. There is nothing specifically "medieval fantasy" about violins and piano, neither instruments are period appropriate. It's exactly same case when piano and violins were and are used to evoke a "medieval fantasy" emotion as these synth sounds are used. Selection is completely arbitrary, and association is only retroactive and overall an illusion. I like that vermintide theme because it conveys a very particular and befitting emotion, one that evokes varying levels of chaos, disorder, panic and discordance, that it doesn't use violins and piano to do so to be medieval means little when the question asked is if it does a theme. The answer is yes, it very clearly has an appropriate theme.
It's also a bit odd to consider that Korsakov's string instruments can evoke the emotions of medieval middle-east fantasy that 1001 nights is supposed to be, but modern synth sounds cannot evoke the emotions of medieval European fantasy isn't it?