It is illegal and Blizzard will probably take legal actions against this project as they did against many other parties providing private servers to the public. The problem is you don't own any right du publish the data you use. Event if you create 100% custom models and textures, it's still Blizzards engine and code.
But nobody can tell if Blizzard will be angered by your project. There are also many private servers existing without any harm by the company.
Usually projects are just shut down. Never heard of a case where ppl had to pay money to Blizzard or went to jail or something.
But they say too that they like custom maps. Are they talking bout WoW or Warcraft?
I'm pretty sure they are talking about Starcraft, Warcraft 3 and Starcraft 2 here due to the fact that they provide custom map editors and custom map publishing mechanisms for these games. I wonder where you took this interview from
