Are there any historical sources about what language Petlura in fact preferred to speak?
Technically that may just be a dialect of Ukrainian, but that distinction gets kind of muddy (specifically political) in Europe. Given some of his known writings, he certainly did know Ukrainian. This could have just been an incident of someone being snobby about his Balachka accent.
As near as I can tell, the concept of Little Russia (essentially Ukrainians being nothing more than an off flavor of Russians, rather than their own people) was a bit of a political marker at the time. In Czarist times, it was a popular viewpoint, but after the revolution that view became discouraged. So it could well be that this particular dismissive attitude toward the local languages/dialects was meant to tell you where on the political spectrum the character Turbin lay.