The reason behind this statement is two words, distributed consensus.
You see Bitcoin speakers, PhD lecturers, and even Bitcoin janitors like Gavin use phrases like this all the time, especially while trying to debunk security models of other systems like proof of stake. The following is one such example from a Gmaxwell link by Andrew Poelstra, created only two months ago titled "Distributed Consensus from Proof of Stake is Impossible":
https://download.wpsoftware.net/bitcoin/pos.pdfThe problem with articles like this isn't the assumption that every theoretical form of proof of stake is broken, we'll just assume that statement is fact for now, the problem is that applying the same scrutiny to the current Bitcoin PoW security model gives you similar results. You really have to go through some mental gymnastics to refer to a system controlled entirely by 1-4 pools as "distributed consensus".
Earlier today, someone insisted to me that technically anything involving two parties counts as distributed. In the case of Bitcoin, using textbook definitions like this obviously doesn't work, since both parties would be at best case 50% each, and attacks can occur with much less hash rate. Defining at what point the Bitcoin network actually is distributed is a tricky thing. For the context of computers, one very important phrase or requirement of distributed computing is the independent failure of components. Some people might argue otherwise, but for my personal defintion, I'm going to argue that the system should still actually function with a single component failure in distributed computing.
Back to the example above, most people would not consider two pools with 50% hashrate a valid consensus model for Bitcoin. Let's assume, only for the sake of example, three pools with 33% each was acceptable. Since we can't allow the network to fall to two pools, due to being an invalid security model, with three pools we are at the lowest state of functionality with no redundancy. Even though the network technically functions in the example, we now have to add another pool, bringing the total to four, just to reach the lowest common definition of distributed computing by having a single increment of redundancy, or how I interpret the textbook definition, "independent failure of components".
Have we really even reached a textbook example of redundancy yet in the example? The answer is an obvious no. If all it takes is two out of four pools in the example to collude to bring us to an invalid security model, we obviously have no redundancy, or independent failure of components. This also happens to be the current state of Bitcoin in the real world. It's not even about Ghash bordering or exceeding 50% hash. Even if they're vastly below that number, the possibility of the top two pools operating under coercion or collusion still means we have no redundancy, or real distributed computing.
Even if you completely ignore the existence of Ghash, the number two and three pools operating together could even qualify as a system with no redundancy under many situations. In other words, people really need to stop praising Bitcoin for it's security because it currently doesn't exist.