This isn't a bug because chains are selected in terms of cumulative difficulty not length of chain. Very quickly a node can distinguish the real chain from the fakes.
Then it is even easier to perform this attack, in theory.
All you would have to do is create a whole bunch of low-difficulty blocks with nearly the same timestamp, then after the "difficulty adjustment" in your branch of the blockchain would result in a super large difficulty. Solve that one block and the blockchain is broken.
Note cumulative, not last.