The movie also mentions that the global population is a classified number, again due to fear of inducing a panic.
The famously blurted line is in the context of the character exhausted and traumatised by their discovery. They aren't acting in the most rational way, just trying to scream their terrible finding to as many people as they can before the authorities catch up. The authories seek to surpress the truth about Soylent Green both because they know many people would react with instinctive disgust to the realisation that they have been unknowing cannibals, and because of what it implies about the sustainability of the food supply: The earth is farmed out, the oceans are dead, and the government has had to resort to the desperate measure of reprocessing the deceased to feed the living.