you are right, seems BTC is loosing decentralization
but that just means... nothing can be decentralised for real
Only primitive organisms. Worms for example.
Ants have primitive centralization. They can build an anthill.
People can work in fully centralized community. They are launching rockets to Mars.
BTC lost its decentralization when some smart guy decided to mine on his video card and another clever guy organized a pool.
The development of bitcoin was never decentralised, looking at it from a wider angle, the blockchain itself is a centralised system(the blockchain) on which we entrust the bitcoin economy. The meaning of decentralisation is that there is not a single entity but the blockchain itself, until there is someone who can make and overtake a mining rig so powerfull it will mine all the remaining blocks and will not share the technology.
This will however result in a non trustworthy blockchain and the whole thing crumbles before the one investing so hard to gain control will have the biggest loss. "Now you're king of the mountain, but it's all garbage!"
I disagree with how he expressed his opinion (doing an attack) about the whole Bitcoin getting centralized but I do agree to some extent that he is right, maybe right now it's not the case but in future the bigger pools will completely take control of the mining operation and then they'll make their own rules and that will again result in a non trustworthy blockchain and maybe these stress test are done by them to increase the miner's fee. I don't think this is how Satoshi envisioned the mining to be, I think it was meant to be completely decentralized and people mining from different parts of the world and not just from few mining farms.