I think the assumption is that American Express could know Delta's plans. They wouldn't be acting based on rumor. However, I think it's more likely that Delta and American Express agreed that the frequency of transfer bonuses had reached the point where the offers were no longer perceived as bonuses, but rather as recurring events.
I agree that American Express would know DL's plans, given that they buy billions of dollars worth of SkyMiles. I also agree that it has more to do with the bonuses becoming too frequent (and perhaps leading to DL's concerns over the glut SkyMiles).
My point regarding the rumor was that there is no substantial evidence of an impending devaluation of SkyMiles. Flyer Talk is fantastic, and I've learned so much from "lurking" and now joining, but the DL forum can be far too speculative.