There is nothing strategic about keeping population of 700 000 000 in the dark about upcoming invasion. Millions could have escaped, even more could have joined defense on the ground. Instead they just left not even informing their own soldiers on the base, not even their best soldiers, Spartans. There is no chance any soldier, or civilian would accept such cowardles leaders.
The thing is we saw Cortana (their most advanced AI) running simulations and giving the same negative results to Ackerson.
You could theorize that letting Reach fall and pretending like it happened 'suddenly and without chance to evacuate' was a strategic choice. That trying to house and feed tens or hundreds of millions who fled Reach would have hampered resources and that 'sacrificing' them actually would allow for the military/leadership to "RALLY THE FLAG" and use the death of nearly the entire population of Reach to have more unilateral control and less opposition in building up their forces to fight back against the Covenant.
Yea. It's detestable and not the path I would have chosen, but I can also rationalize that it's possible there was an 'outline/path' followed based on simulations and 'game theory' on it being a worthwhile move.