This 51% attack can occur with any cryptocurrency. Lisk, as with all other cryptocurrencies, is fully dependent on their base users.
If the majority (> 50%) of Lisk owners choose to use unregulated Somalian exchanges to trade, it is the end-users fault if the unregulated Somalian exchange decides to control the Lisk network by using end-user funds for voting their own delegates into the active delegation.
Fortunately, exchanges are fairly regulated these days. These exchanges do not use funds for voting or staking.
It doesn't have to be an exchange.
Also, it's not a 51% attack. It is a 100% attack by a minority holder. A minority holder can control 100% of the security mechanism. That can't be said of Bitcoin or any PoS system.
The vote is equivalent to the amount of coins in your wallet (e.g one account with 51,000,000 LISK has a vote presence of 51% of the possible 100% votes). A minority holder cannot gain control of all the 101 delegates, that is unless no one decides to vote.
Holding 51% of the total coin supply is the only guaranteed way to take control of the network.
That's not how I understand it. Each wallet gets 101 votes. If a wallet has a plurality of the weight, it can vote for its own nodes with that plurality.
What you say would be true if each wallet got only 1 vote. A big wallet could vote for itself or another single wallet once and the blockchain security would be split accordingly.
With delegated staking, it would take a coordinated effort to undermine the big wallet.
All other block chain security systems rely on the opposite, which is a lack of coordination.
