Ehh, not a fan of the theory desu.
It takes away from the nobility of an alien race defending Earth. Suddenly they're not doing it for freedom, or to protect innocent lifeforms, but because the planet is basically manipulating them. Beyond that, why did only some gems get affected by this?
The cluster also had no indication of being changed or manipulated in any way when Steven first arrived. The shards all had the same need to form and search for themselves that every other gem mutant had. All changes were initiated by Steven.
The idea that the diamonds are purely logical seems like a pretty obvious propaganda move by the diamonds. Even in her short conversation with Peridot, Yellow Diamond showed very visceral and emotional reaction to what Peridot was saying, something a purely logical being would not do.
The second to last paragraph seems like it has a sentence or 2 cut off from the beginning of it. My best guess as to what it's saying is Earth gave Steven his extra powers, but if that were the case, why haven't the other gems been affected by the same gift? Arguably you could say Ruby and Sapphire's ability to fuse came from the same source, but what about Pearl and Amethyst? Why does Pearl only receive a temperament change? She said her ability comes from hard work and practice, why don't Steven's?
I suppose it's a plausible theory, nothing directly contradicts it, but it seems highly unlikely to me.