He actually avoided it for a long time. He had suspicions it would be worthwhile for an industry-wide effort to make a games/high performance-specific language, but thought it would be too much work for an individual developer.

While building a research prototype though, he decided it was easier than he thought it would be. While he has been finishing The Witness (the subject of the original posting) he has been prototyping the language and it has been going well enough he decided he won't build another game in C++.