Unfortunately this is one of those things where it falls into a rather wide spectrum of "possible" scenarios for which we have no practical way to rank-sort and downselect to a smaller subset of "plausible" scenarios owing to the lack of information.
What you propose is most certainly possible. There is nothing about the engineering constraints and boundary conditions that would out-right preclude such a scenario.
I know for fact that in this industry competitive products and info on them do feed back into the binning decision cycle from a cost-benefits perspective that includes everything under the sun from production costs to field-fail warranty liabilities and so on.
But there is also nothing to go on here to suggest that Nvidia did not decide 24 months ago that the launch clocks of the 590 were going to be anything but their current stock-clocks.
Just going from experience in how I've seen these things play out while being an engineer on that other side of the fence, the timeline of events involved here, the specific situation of power-management issues on both sides of the fence, and adding a heavy amount of "personal opinion and WAG" I am personally inclined to believe that Nvidia targeted the 590 to fit within the prior existing 300W threshold.
And when the HD6990 was launched (whether Nvidia was dragging their feet for that to happen before the 590 was released is irrelevant) at its respective pricepoint, performance point, and power point, Nvidia scrambled to assess and determine whether an engineering (data) driven business decision could be made to justify pushing up the clockspeed and power-consumption of the 590 so that it was in the same performance/power regime as the HD6990 without resorting to a complete redesign of the reference PCB as well as avoiding the use of even more expensive components that would push the BOM even higher still.
Just my opinion, but bringing everything I have to bear on the subject (experience, education, intuition, etc) this "story" just fits all the data points with the least amount of trade-offs, wild suppositions and intractable boundary conditions.
In short - IMHO
Occam's Razor strongly supports the notion you put forth in your post.