The essay isn't about what matters in the absolute. It's about deciding where to spend a marginal hour, year, or life with imperfect information. And it's obviously true in retrospect that Newton's lifetime was spent in a way which produced more good to society than yet another theological life. Contrast his contributions to the most prominent 17th century theologians:
John Milton is on that list. Newton was exceptionally important, but our world would be poorer without Paradise Lost. I'm glad Milton spent his time as he did.
https://en.wikipedia.org/wiki/List_of_Christian_theologians#...