How is "fifty million man-years" development effort inside <The Art of Unix Programming>

I was reading the book There is one line claiming >The three and a half decades between 1969 and 2003 is a long time. Going by the historical trend curve in number of Unix sites during that period, probably somewhere upwards of fifty million man-years have been plowed into Unix development worldwide. That's (50million man-year / 35 year) = (1.47 million man-year / year) goes into Unix development during that period. It means 1.47 million developers worked on Unix system yearly. Personally I find the number a bit hard to believe. Or am I understanding the number in the wrong way?