You're missing an important thing - that Go is trivial to simulate. AG can play millions of self games relatively cheaply. A human-level intelligence would need to run trials on a simulation of the real world, which is impossible to create as of yet. That is why the author was insisting on the environment (the world or a simulator) - that the environment is the bottleneck. You can think of the environment as a dynamic dataset. You know that data is crucial in AI, thus, the lack of sufficiently complex data would hamper AGI.

The fact that there is no "universal environment" means there can be no general intelligence. There can be just environment specific intelligences (situated intelligence, as the author said). The concept of AGI is just a reification of narrow AI - an illusion, there is no such thing.