I think that first reason is the most telling -- establishing credibility. This guy was a head honcho of the ACM, the highest-profile professional organization I know of in computing. (Apparently very weak in comparison to what some other professions have.)
Incidentally, the ACM has a bad reputation among some: http://www.nhplace.com/kent/PFAQ/acm.html
As for his substantive points, I don't see why math is any different. After all, scientists make frequent use of math. But maybe I misunderstand.
But of course, an individual may pursue computing as a science. I vaguely recall that in his accessible marxist intro to math, Kolmogorov treated math like a science. Not to mention those who thought they'd find god through math. (Gödel, maybe?)
Tayssir