Obviously this statement would be completely false if the word "thoughtful" were left out. However, I'm sure that the definition of "thoughtful" in Singer's view is quite different than what you'd find in your copy of Webster's. It obviously refers to the few percent of the population that shares Singer's "enlightened" views.
“...most thoughtful people...”
That’s a classic logical fallacy, a bandwagon appeal. In other words, you aren’t thoughtful if you don’t believe what Singer and “thoughtful” people believe.