Sure, the retort's point is strong that it's impossible for four pounds of a human's bodyweight to be composed of forty trillion bacteria, however, the rebuttal has missed the bigger picture. Nikhil was merely attempting to open up the reader's eyes to the overwhelming importance of bacteria in creating the human body. You see, Nikhil wanted readers to understand that even though bacteria make up a small fraction of a human's total body mass, they are still an important part of our physiology.
The retort speaks to the fact that other cells such as epithelial cells, fat cells, connective tissue and muscle cells make up the bulk of a human body. But does that mean Nikhil was incorrect in saying forty trillion bacteria contribute four pounds? No! He was simply using the number of bacteria as a metric to measure the impact of those billions of little microbes in our bodies.
While it's true that bacteria are incredibly tiny and many trillions of them would not equate to four pounds, they still have a profound effect on our biology. Not only do they aid in digestion, but they can also help control inflammation and strengthen our immune systems. So even though their mass may be negligible, their influence is powerful.
To sum it up, the retort failed to recognize the value in Nikhil's essay: to marvel at how much bacteria contribute to humans despite their small mass. And perhaps that's why it's UrRong in its criticism of Nikhil!