Q̶u̶a̶d̶r̶a̶t̶i̶c̶ voting can be bypassed if whales can make a large amount of identities. This has nothing to do with quadratic or not. As long as the voting system satisfies the non dictatorship property of Arrows theorem they are vulnerable to Sybil.
I think what OP meant is that if you have an organisation with 100 votes or 100 organisations with 1 vote there's no difference in linear, but there's a huge difference in quadratic.
Quadratic voting can be bypassed if whales can make a large amount of identities. The lack of strong identity and Sybil attacks is the biggest unsolved problem in distributed systems of the free-to-join class.