The argument is that the console versions has already had an effect on the design of the game some time during development. The decision to make versions for consoles meant that several aspects of the game had to be redesigned so that it can accommodate console gamers and as such, it resulted in the game being "dumbed down".
This particular argument is certainly making its rounds. And, in most cases, it's being used to validate the viewpoints of people who disagree with how Diablo III was designed. While having those disagreements is fine, blaming console for why they even exist is a pretty poor scapegoat.
No, the PC version of Diablo III was not designed for console from the beginning. It was developed for the PC with one major goal in mind: provide players with an awesome hack-and-slash computer game that's both fun and engaging. Whether or not we achieved that goal is certainly up for debate within this community, but it doesn't make it any less real or valid. All the decisions we made when developing Diablo III -- including combat, skills, character controls, user interface, and itemization -- were ones we believed would deliver a great online experience, and the same can be said for the improvements we've made since launch. These decisions were not hindered (i.e. "dumbed down") by the development of a PlayStation version or altered to accommodate it.
Many articles tend to differ with you. Remember these two?
Sept. 2011 Jay Wilson is exploring console version
Jan 2012 Bashiok confirms console versions
I imagine that considering this is when we heard about it that this was discussed much earlier, probably early enough to have a significant impact on how the game was developed, even the PC version (mid 2010-ish as people are mentioning probably).
Of course, they denied it later on without actually outright denying it:
My belief is they began to deny out of fear of what is happening now: people believe the PC game's development was hindered by attempts to bring it to console. Denying it allowed them to respond with the above quote and similar others.
My belief is that Blizzard had plans well in advance to bring this to console, and that these plans did, in fact, affect development.
Also, as people have pointed out, of course it wasn't developed from the beginning to be for console. That doesn't mean the decision to bring it to console had no effect on development.
Edit: I also wanted to point out how funny it is that Runic Games, some of whom worked on D2, took Torchlight in the opposite direction. The first game was for both PC and Console and had no multiplayer at all but they made the second game PC only with multiplayer. Unlike Blizzard, though, they allowed for both online and offline play. they allowed modding, and they allowed more than 4 people in a game (likely a product of plans for a console version for D3). This is among many other things they changed and innovations they made for the better.