does it mean 51-attack will happen if someone buy more than 51% BTC
No. A 51% attack refers to mining capacity. Owning bitcoins doesn't give you any power or control over Bitcoin (except perhaps the exchange rate).
A 51% attack would be possible if someone bought up 51% of the bitcoin and we decided to change the mining method to PoS
Exchanges are already capable of this attack if it shifts to POS. The amount of machines dedicated to Mining is very much. If it were to shift to POS, huge losses will be suffered.