Is there any evidence that it actually uses Markov chains? I don't see any mention. The title here might as well be made up to troll HN?
For example, it is not necessarily trying to beat you. This was inspired by an experience with https://luduxia.com/whichwayround/ where I found most traffic came from people trying to break it, until one day it hit a mailing list and exploded.
This one I updated the game a bit, submitted a link, and it went nowhere when I posted it some days ago. I woke to a Linode alert of sustained outgoing traffic and it turns out the post had been resurrected.
In any case Markov chains are a simple and elegant mechanism for making predictions, easy enough to understand so that even school kids at grade eleven (which is where probability theory got introduced when I underwent it) can follow the popular "weather forecasting" tutorial using MCs.
Onwards, Hidden Markov Models (HMMs) add to MCs a layer of hidden states and associated emission probabilities. To learn these, check out Lawrence Rabiner's beautiful tutorial: https://ieeexplore.ieee.org/document/18626
Apart from their simplicity and mathematical elegance it is remarkable how little data and electricity these models require to do a good job.