You didn't answer the question. A miner holding 60% of hashing power could pretend to be 10 independent miners with 6% power each.
Yes he could.
However the assumption is that if he either
A. Started to do doublespends via chain rollbacks.
B. Started to only allow his own blocks.
It would become obvious that these 10 fake pools were in fact colluding and people would remove them from their trust lists and start to delay them/reject them.
(The clients today already have automatic alarms for 2+ blockchain forks I believe)
In other words one single honest entity could control 100% of the mining power in my system and no one would necessarily know about it.
However as long as no attacks are taking place we don't care.