Blatantly contradicted by this.
http://www.halopedia..._of_Reach_1.jpg
Halsey's theory is an in-universe theory, not fact. The Covenant do burn the surface of a world to the point where it cannot support life anymore. What has been changed is that they don't burn every square centimeter of the planet. Here are the changes.
"When the Covenant destroys a planet, they typically move their large warships closer and blanket the world with a series of crisscrossing orbits to ensure that every square millimetre of the surface is covered with plasma bombardments."
is changed into:
"When the Covenant destroys a planet, they typically move their large warships closer and blanket the world with a series of crisscrossing orbits to ensure that nothing could ever survive on its surface."
"Before I jumped to Slipspace, I witnessed the poles destroyed, and approximately two thirds of the planet's surface was on fire."
is changed into:
"Before I jumped to Slipspace, I witnessed the poles destroyed, and a significant portion of the planet's surface was on fire."
And lastly
"Something didn't fit. Covenant plasma bombardment had always proceeded in an orderly crisscrossing pattern across a planet until every square centimeter of the surface was glass and cinder."
is changed to:
"Something didn't fit. Covenant plasma bombardment had always proceeded in an orderly crisscrossing pattern across a planet until everything on its surface was glass and cinder."
So to reiterate the Covenant still glass worlds, its just not to such an extreme degree that they destroy every square centimeter.
You bring up some valid points, but for all we know Reach may be a hell of a lot smaller than earth.
And the majority of the covenant navy was engaged at reach.