Excerpt: "Following Defcon we had a series of conversations with the Hearthstone team about our research... they were very concerned that our real time dashboard that can predict your opponent’s deck will break the game balance by giving that person (that is, whoever has the tool) an unfair advantage. They also expressed concern that such a tool..."
There's no evidence his utility was derived from reverse engineering or a TOS violation. It appears to be based on evaluating a large number of Hearthstone matches and using those results to create a mathematical model that lets you find "undervalued" cards. One application of the model is a utility that can help to predict "what your opponent is going to play based on previous turns." https://www.elie.net/blog/hearthstone/how-to-appraise-hearth...
Blizzard has taken aggressive legal action before. As EFF tells it, Blizzard filed "a DMCA lawsuit against a group of volunteer game enthusiasts who created software that allowed owners of Blizzard games to play their games over the Internet. The software, called "bnetd," allowed gamers to set up their own alternative to Blizzard's own Battle.net service." https://www.eff.org/wp/unintended-consequences-under-dmca
This time, I wonder if the decision not to release the utility was prompted by a friendly query from a Blizzard developer or a C&D nastygram by a Blizzard attorney -- with a lawsuit threatened if he didn't comply.