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.
Let me first re-direct you to the Lisk Delegate Handbook:
https://lisk.io/documentation?i=lisk-handbooks/DelegateHandbookThere are some discrepancies in your understanding of how voting works, I would advise to read the recently drafted Delegate Handbook for a detailed explanation.
Now, to explain it as best as I can in my own words.
Your account has a voting presence that equals the amount of funds you have in your wallet. The more funds you have in your wallet, the higher voting presence it has. This account can vote for 101 different delegates, you cannot stack voting. Here's an example:
John's account has 15M LISK, he has a voting presence of 15% (out of the possible voting presence total of 100%)
James account has 10M LISK, he has a voting presence of 10% (out of the possible voting presence total of 100%)
If John and James were to vote on delegates, they would account a total of 25% vote presence on the chosen delegates. Keep in mind again that they may not vote twice on the same account, they must either a) not vote b) vote on the desired accounts only.
A simpler way to view this is, if one person holds all of the funds (100M), he has 100% voting presence, and has complete dictatorship over who will be the 101 active delegates. However, as stated previously, you don't need 100%, 51% is enough to rule the other half of voters.
This brief explanation may not be the best, so I would advise you to read the Delegate Handbook thoroughly. Your concerns should be answered.