In confounding human speech (Gen 11:7), is God not doing something he temporarily undid at the Pentecost?

Yes. At the Pentecost, “they were all filled with the Holy Ghost, and began to speak with other tongues.” (Acts 2:4) It was then that “devout men, out of every nation under heaven” (2:5) heard the speech and wondered. In fact, many nations are listed, and people present from those nations did “hear them speak in our tongues the wonderful works of God.” (2:9-11) This was precisely the opposite of what happened at Babel, where God confused languages to prevent the accomplishment of the vain and ultimately (in this case, anyway) evil works of man.