OK, yeah I think I understand now. Thanks!
I would think putting "Markov Chain" in the name implies some amount of probability, but maybe that's why I'm not in charge of naming things.
Viewing post in Sibyl - The Procedural Level Painter jam comments
At least to my understanding, the Markov Chain variant uses randomness (like regular WFC) but it doesn't use weighted decisions. So it asks "what could go here" while Sibyl asks "what would best fit here".
All that said, WFC and MkWFC are still very good algorithms. Most importantly, they're naturally faster, so they can be used in games for real-time generation, while Sibyl sacrifices some speed for control and quality. I am still working to optimize Sibyl, but it does have a higher workload to contend with.