I think the authors are saying that the number of mental models (possible worlds) that language can suggest is unmanageably infinite, unless the listening brain

I think the authors are saying that the number of mental models (possible worlds) that language can suggest is unmanageably infinite, unless the listening brain is capable of rapidly eliminating all the infeasible / implausible worlds before forming a viable model of what the speaker is thinking, before formulating a response. To do that, the listener needs to bring to bear a wealth of world knowledge and common sense, all of which is lacking in the current models of AI created by deep learning. As such, unless AI can somehow build useful models of the world and combine them with basic horse sense, it will always be just a one trick pony.